changes ovs agent to get bridges via ovs_lib

ip_lib is currently used to list the bridges
in the Open vSwitch neutron agent.

use of ip_lib blocks reuse of the Open vSwitch agent
with userspace only open vSwitchs implementations.

this patch replaces calls to ip_lib with ovs_lib.get_bridges

Closes-Bug: #1331569
Change-Id: I5935d39b314055063a64266bda0cc4c2d1ac05fc
Merge "changes ovs agent to get bridges via ovs_lib"