Improve ovs_lib bridge management

Add the ability to add and remove bridges, check for bridge
existence, and lookup the bridge associated with a port.

This change is in support of functional testing for an ovsdb
monitor.

Partial-Bug: #1177973

Change-Id: I419923d8d77983997cd347fcf063b0bc367c0bbc
Merge "Improve ovs_lib bridge management"