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
(cherry picked from commit 010bd1f392e67a6fcd276593b8c79acfe41d1cc7)
Update send_arp_for_ha default in l3_agent.ini

The default for send_arp_for_ha was 3 and is now 0.
This patch changes the comment in l3_agent.ini to
show the new default.

Change-Id: I59a807105d71d1118fd9ae586a6c9a07995f81cd
Closes-Bug: #1292293
(cherry picked from commit 4a6e77e316cf50761b1ee07be63e490f29519711)
Merge "Update send_arp_for_ha default in l3_agent.ini" into stable/havana
Merge "Improve ovs_lib bridge management" into stable/havana