This patch fixes an error that does not result in a test failure but
causes the following error to be logged in the test logs:
TypeError: <MagicMock name='LinuxBridgeManager().local_ip' id='666599248'>
is not JSON serializable
Related-Bug: #1234857
Change-Id: I65cdd997745d8b760031f8f32e3c10682ad2288e