[2.1] regiond should not rely on maas-rack command

[Nobuto Murata]

If I understand it correctly, MAAS regiond can be deployed without rackd in the same host. However maas-regiond relies on maas-rack command which is in maas-rack-controller package and gives traceback in some critical tasks such as writing /etc/ntp/maas.conf.

2017-04-05 02:37:53 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: