bgpvpn: agent extension, fix port association 'routes' update
bgpvpn: agent extension, fix port association 'routes' update

This change fixes the code to handle port association updates:
if an update adds routes and removes routes, we were
only detaching removed routes.

Change-Id: Ib8c7d3162d5322702ad6a149f903b1c705daabd8
Closes-Bug: 1743574

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