Setup MPLS bridge only if OVS agent
Setup MPLS bridge only if OVS agent

Agent code is common with modified openvswitch agent
for bgpvpn driver, but the OVS part should (setting
up the mpls bridge) should only be done if this code
is ran inside the OVS agent, and not for the linuxbridge
agent

Change-Id: Icdc6e3ca40d2c0b2b7fab414d6fdb27c7542ddf0

stackforge/networking-bagpipe-l2,openstack/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2