MPLS OVS dataplane: simplify OVS rule cleanup
MPLS OVS dataplane: simplify OVS rule cleanup

Simplify OVS rule cleanup at VRF teardown, by relying on cookies.

This avoids a we currently have in VPNInstance.cleanup: artificial
route withdrawal events that we get from RouteTableManager may fail
to be received before the eventloop that processes them is stopped.

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