Remote Drive's CapacityGiB should be greater or equal to Master Drive's CapacityGiB

When trying to compose RSD nodes with a remote storage size smaller than the Master's size we get a stacktrace and an error.

MAAS API command:

# maas admin pod compose 91 storage="root:10(local),newell:2(iscsi)"
Unable to composed machine because: Failed to complete pod action: Remote Drive's CapacityGiB [1.862645149230957GiB] should be greater or equal to Master Drive's CapacityGiB [10GiB].

Stacktrace:

==> /var/log/maas/rackd.log <==
2017-04-24 19:46:07 provisioningserver.rpc.pods: [critical] pmatulis: Failed to compose machine: RequestedMachine(hostname='fresh-bedbug', architecture='amd64/generic', cores=1, memory=1024, block_devices=[RequestedMachineBlockDevice(size=10000000000, tags=['local']), RequestedMachineBlockDevice(size=2000000000, tags=['iscsi'])], interfaces=[RequestedMachineInterface()], cpu_speed=None)

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 972, in compose
    raise PodActionError(message)
provisioningserver.drivers.pod.PodActionError: Remote Drive's CapacityGiB [1.862645149230957GiB] should be greater or equal to Master Drive's CapacityGiB [10GiB].