Same problem when running maas-rack controller on a second machine:

Intel NUC 5i5MYHE RACK CONTROLLER: /var/lib/maas/maas.log
Feb 15 19:58:13 rack2-maas-rack0 maas.import-images: [info] Updating boot image iSCSI targets.
Feb 15 19:58:13 rack2-maas-rack0 maas.boot_image_download_service: [error] Failed to download images: Command `sudo /usr/sbin/tgt-admin --conf /var/lib/maas/boot-resources/current/maas.tgt --update ALL` returned non-zero exit status 2:#012Config file /var/lib/maas/boot-resources/current/maas.tgt not found. Exiting...
Feb 15 19:58:17 rack2-maas-rack0 maas.dhcp.probe: [error] Unable to probe for DHCP servers: Connection was closed cleanly.

tgtd dies, rendering the rack controller unusable since it cannot complete the maas-region import.

Raspberry Pi 3B REGION CONTROLLER:  /var/lib/rediogd.log

2017-02-15 15:06:02 twisted.python.log: [info] ::ffff:192.168.199.6 - - [15/Feb/2017:20:06:01 +0000] "GET /MAAS/rpc/ HTTP/1.0" 200 316 "-" "provisioningserver.rpc.clusterservice.ClusterClientService"
sudo2017-02-15 15:06:28 provisioningserver.utils.services: [info] Neighbour observation process for enxb827eb208904 started.
: a password is required
2017-02-15 15:06:26 maasserver.regiondservices.ntp: [critical] Failed to update NTP configuration.

Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 824, in worker
    return target()
  File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
    task()
  File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
    task()
--- <exception caught here> ---
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 857, in callInContext
    return func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 225, in wrapper
    result = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/provisioningserver/ntp/config.py", line 49, in configure
    mode=0o644)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/fs.py", line 281, in sudo_write_file
    raise ExternalProcessError(proc.returncode, command, stderr)
provisioningserver.utils.shell.ExternalProcessError: Command `maas-rack atomic-write --filename /etc/ntp/maas.conf --mode 0644` returned non-zero exit status 1:

/var/lib/rackd.log

2017-02-10 15:42:28 sstreams: [info] maas:v2:download/maas:boot:ubuntu:amd64:ga-16.04:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:42:31 provisioningserver.rackdservices.image_download_service: [critical] Downloading images failed.

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 434, in errback
    self._startRunCallbacks(fail)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 588, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1184, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/rackdservices/image_download_service.py", line 128, in maybe_start_download
    yield self._start_download()
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/rackdservices/image_download_service.py", line 115, in _start_download
    get_proxy_url("https"))
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/rpc/boot_images.py", line 145, in _import_boot_images
    imported = yield deferToThread(_run_import, sources, **proxies)
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 225, in wrapper
    result = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/provisioningserver/rpc/boot_images.py", line 119, in _run_import
    imported = boot_resources.import_images(sources)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/boot_resources.py", line 308, in import_images
    update_iscsi_targets(snapshot_path)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/boot_resources.py", line 238, in update_iscsi_targets
    update_targets_conf(snapshot_path)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/boot_resources.py", line 198, in update_targets_conf
    service_monitor.ensureService("tgt").wait(30)
  File "/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 231, in wait
    result.raiseException()
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 368, in raiseException
    raise self.value.with_traceback(self.tb)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/service_monitor.py", line 535, in _ensureService
    state = yield self.getServiceState(service.name, now=True)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/service_monitor.py", line 251, in getServiceState
    active_state, process_state = yield self._loadServiceState(service)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1128, in _inlineCallbacks
    result = g.send(result)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/service_monitor.py", line 478, in _loadSystemDServiceState
    service.service_name, active_state))
provisioningserver.utils.service_monitor.ServiceParsingError: Unable to parse the active state from systemd for service 'tgt', active state reported as 'deactivating'.
2017-02-10 15:42:52 Uninitialized: [info] ClusterClient connection established (HOST:IPv6Address(TCP, '::ffff:192.168.199.2', 33588) PEER:IPv6Address(TCP, '::ffff:192.168.199.2', 5253))
2017-02-10 15:42:53 provisioningserver.rpc.clusterservice: [info] Event-loop 'juju-rack2:pid=7317' authenticated.
2017-02-10 15:42:57 provisioningserver.rpc.clusterservice: [info] Rack controller 'm4t3ww' registered (via juju-rack2:pid=7317).
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:centos:amd64:generic:centos70: to_add=['20161201_01'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:armhf:hwe-16.04-edge:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:grub-efi-signed:amd64:generic:uefi: to_add=['20170125.0'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:amd64:hwe-16.04:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:grub-ieee1275:ppc64el:generic:open-firmware: to_add=['20170125.0'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:amd64:hwe-16.04-lowlatency-edge:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:grub-efi:arm64:generic:uefi: to_add=['20170125.0'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:amd64:hwe-16.04-edge:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:armhf:hwe-16.04:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:amd64:hwe-16.04-lowlatency:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:armhf:generic-lpae:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:amd64:ga-16.04-lowlatency:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:pxelinux:i386:generic:pxe: to_add=['20160930.0'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:armhf:ga-16.04:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 sstreams: [info] maas:v2:download/maas:boot:ubuntu:amd64:ga-16.04:xenial: to_add=['20170207'] to_remove=[]
2017-02-10 15:43:27 twisted.internet.defer: [critical] Unhandled error in Deferred:
2017-02-10 15:43:27 twisted.internet.defer: [critical] 

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 434, in errback
    self._startRunCallbacks(fail)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 588, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1184, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/rpc/boot_images.py", line 145, in _import_boot_images
    imported = yield deferToThread(_run_import, sources, **proxies)
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 225, in wrapper
    result = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/provisioningserver/rpc/boot_images.py", line 119, in _run_import
    imported = boot_resources.import_images(sources)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/boot_resources.py", line 278, in import_images
    update_iscsi_targets(os.path.join(storage, 'current'))
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/boot_resources.py", line 238, in update_iscsi_targets
    update_targets_conf(snapshot_path)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/boot_resources.py", line 198, in update_targets_conf
    service_monitor.ensureService("tgt").wait(30)
  File "/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 231, in wait
    result.raiseException()
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 368, in raiseException
    raise self.value.with_traceback(self.tb)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/service_monitor.py", line 535, in _ensureService
    state = yield self.getServiceState(service.name, now=True)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/service_monitor.py", line 251, in getServiceState
    active_state, process_state = yield self._loadServiceState(service)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1128, in _inlineCallbacks
    result = g.send(result)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/service_monitor.py", line 478, in _loadSystemDServiceState
    service.service_name, active_state))
provisioningserver.utils.service_monitor.ServiceParsingError: Unable to parse the active state from systemd for service 'tgt', active state reported as 'deactivating'.

/var/lib/maas.log

Feb 15 15:04:30 juju-rack2 maas.import-images: [info] Downloading image descriptions from http://images.maas.io/ephemeral-v3/daily/
Feb 15 15:04:43 juju-rack2 maas.refresh: [info] Refreshing rack controller hardware information.
Feb 15 15:04:48 juju-rack2 maas.rpc.rackcontrollers: [info] Existing rack controller 'rack2-maas-rack0' has connected to region 'juju-rack2'.
Feb 15 15:04:51 juju-rack2 maas.bootsources: [info] Updated boot sources cache.
Feb 15 15:04:51 juju-rack2 maas.bootresources: [info] Started importing of boot images from 1 source(s).
Feb 15 15:04:51 juju-rack2 maas.import-images: [info] Downloading image descriptions from http://images.maas.io/ephemeral-v3/daily/
Feb 15 15:04:55 juju-rack2 maas.bootresources: [info] Importing images from source: http://images.maas.io/ephemeral-v3/daily/

...

Feb 15 15:05:05 juju-rack2 maas.interface: [info] eth0 (physical) on rack2-maas-rack0: New MAC, IP binding observed: 00:16:3e:2d:57:28, 192.168.199.6

...

Feb 15 15:05:19 juju-rack2 maas.node: [info] juju-rack2: Storage layout was set to flat.



New rack controller = intel NUC 5i5MYHE 

root@rack2-maas-rack0:/var/log/maas# cat /etc/issue
Ubuntu 16.10 \n \l

root@rack2-maas-rack0:/var/log/maas# uname -a
Linux rack2-maas-rack0 4.8.0-37-generic #39-Ubuntu SMP Thu Jan 26 02:27:07 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

instead of another Pi device just to see if the behavior was any different on the other architecture.