RSD composition with remote storage fails due to AttributeError

[Newell Jensen]

Composing an RSD node with remote storage fails with an AttributeError:

# maas rsd pod compose 104 cores=40 cpu_speed=2000 memory=7812 architecture="amd64/generic" storage="root:10(local),remote:30(iscsi)"

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/drivers/pod/rsd.py", line 1057, in compose
    remote_drives, logical_drives, targets, request)
  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/drivers/pod/rsd.py", line 722, in get_pod_machine
    targets, discovered_machine, request=request)
  File "/usr/lib/python3/dist-packages/provisioningserver/drivers/pod/rsd.py", line 607, in get_pod_machine_remote_storages
    if block_device.iscsi_target == target_name:
builtins.AttributeError: 'RequestedMachineBlockDevice' object has no attribute 'iscsi_target'