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 "Add the option to minimize ovs l2 polling"