Nova should pass device_id='' instead of None to neutron.update_port()

In the case that neutron raises an error and nova needs to undo the api
calls it's done against neutron it should pass device_id='' instead of
None otherwise neutron raises an exception.

Note: This should have a unit test but it's near impossible
to add one right without duplicating all of _stub_allocate_for_instance
due to the fact that it's using some crazy logic in the tests to make
decisions based on an index that's passed it. X.x

Change-Id: I91334c07401ff379f71fdef1d5bd080cc68a72ab
Closes-bug: #1285886
Merge "Nova should pass device_id='' instead of None to neutron.update_port()"