bgpvpn: add 'local_pref' to OVO port routes
bgpvpn: add 'local_pref' to OVO port routes

Adds 'local_pref' to the BGVPN OVO object for
PortAssociationsRoute.

Change-Id: Iecced4718be4ee84651f9886271285bcf7df164e
Partially-Implements: blueprint routes-control

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