[2.6, master] Deploying a machine with a bridge on the PXE interface breaks the deployment

Attempting to deploy a machine with an interface configured as a bridge fails and fallbacks to the enlistment environment.

To reproduce:

1. Create a machine with 1 nic
2. Deploy the machine
3. Release the machine
4. Configure a bridge in the machine
5. Attempt to deploy, and you will see this traceback:

==> /var/log/maas/regiond.log <==
2019-02-27 11:04:14 provisioningserver.rpc.common: [critical] Unhandled failure dispatching AMP command. This is probably a bug. Please ensure that this error is handled within application code or declared in the signature of the b'GetBootConfig' command. [
maas-bionic:pid=386:cmd=GetBootConfig:ask=173]
        Traceback (most recent call last):
          File "/usr/lib/python3/dist-packages/twisted/internet/asyncioreactor.py", line 267, in run
            self._asyncioEventloop.run_forever()
          File "/usr/lib/python3/dist-packages/twisted/internet/asyncioreactor.py", line 290, in run
            f(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 500, in errback
            self._startRunCallbacks(fail)
          File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 567, in _startRunCallbacks
            self._runCallbacks()
        --- <exception caught here> ---
          File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 653, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/usr/lib/python3/dist-packages/twisted/protocols/amp.py", line 1171, in checkKnownErrors
            key = error.trap(*command.allErrors)
          File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 359, in trap
            self.raiseException()
          File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 385, in raiseException
            raise self.value.with_traceback(self.tb)
          File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 250, in inContext
            result = inContext.theWork()
          File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
            return func(*args,**kw)
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 894, in callInContext
            return func(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 235, in wrapper
            result = func(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 756, in call_within_transaction
            return func_outside_txn(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 563, in retrier
            return func(*args, **kwargs)
          File "/usr/lib/python3.6/contextlib.py", line 52, in inner
            return func(*args, **kwds)
          File "/usr/lib/python3/dist-packages/maasserver/rpc/boot.py", line 351, in get_config
            machine.boot_interface = machine.interface_set.get(mac_address=mac)
          File "/usr/lib/python3/dist-packages/django/db/models/manager.py", line 85, in manager_method
            return getattr(self.get_queryset(), name)(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/django/db/models/query.py", line 384, in get
            (self.model._meta.object_name, num)
        maasserver.models.interface.MultipleObjectsReturned: get() returned more than one Interface -- it returned 2!

==> /var/log/maas/rackd.log <==
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [critical] TFTP back-end failed.
        Traceback (most recent call last):
        Failure: twisted.protocols.amp.UnhandledCommand: (b'UNHANDLED', 'Unknown Error [maas-bionic:pid=386:cmd=GetBootConfig:ask=173]')

2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/C0A87A09 requested by 192.168.122.9
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/C0A87A0 requested by 192.168.122.9
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/C0A87A requested by 192.168.122.9
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/C0A87 requested by 192.168.122.9
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/C0A8 requested by 192.168.122.9
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/C0A requested by 192.168.122.9
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/C0 requested by 192.168.122.9
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/C requested by 192.168.122.9
2019-02-27 11:04:14 provisioningserver.rackdservices.tftp: [info] pxelinux.cfg/default requested by 192.168.122.9
2019-02-27 11:04:15 provisioningserver.rackdservices.http: [info] /images/ubuntu/amd64/ga-18.04/bionic/daily/boot-kernel requested by 192.168.122.9
2019-02-27 11:04:15 provisioningserver.rackdservices.http: [info] /images/ubuntu/amd64/ga-18.04/bionic/daily/boot-initrd requested by 192.168.122.9