lb-agent: fix get_interfaces_on_bridge returning None

Change get_interfaces_on_bridge() to return [] instead of None
if the bridge is not found, as callers are expecting an iterator.

Change-Id: I359a2a0cf932c049f6891d4ba3eb8b9b23ab360d
Related-Bug: 1284311
Remove mock.patch.stop from tests that inherit from BaseTestCase

The tests that inherit from BaseTestCase don't need to stop their
patches, since this is already done in the base class

Change-Id: Ibb1183e521686d6e948046997b32f4044d91d9e7
Closes-bug: #1305656
Merge "Remove mock.patch.stop from tests that inherit from BaseTestCase"
Merge "lb-agent: fix get_interfaces_on_bridge returning None"