Fix re-creation of the pool directory

Whenever vip is deleted the complete pool directory
is deleted, but since pool is still alive the get_stats
function recreates the directory. The fix prevents the
re-creation of the directory. Respective unit test case
is also modified

Change-Id: I577e8e3a51db361210aa83a6c7f9b4f61210e924
Closes-Bug: #1317871
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 "Fix re-creation of the pool directory"
Merge "changes ovs agent to get bridges via ovs_lib"