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
Add the option to minimize ovs l2 polling

This change adds the ability to monitor the local ovsdb for
interface changes so that the l2 agent can avoid unnecessary
polling.  Minimal changes are made to the agent so the risk
of breakage should be low.  Future efforts to make the agent
entirely event-based may be able to use OvsdbMonitor as a
starting point.

By default polling minimization is not done, and can only be
enabled by setting 'minimize_polling = True' in the ovs
section of the l2 agent's config file.

Closes-Bug: #1177973

Change-Id: I26c035b48a74df2148696869c5a9affae5ab3d27
Merge "Improve ovs_lib bridge management"
Merge "Add the option to minimize ovs l2 polling"