BGPVPN Network association does not work if subnets have routers

This is a regression compared to Be with vpnservice.

The most minimal setup I could reproduce this with was:

-Two networks and a subnet for each
-Create two routers and connect each to one subnet
-Create an instance on each subnet
-Pinging instances does not work at this step as it should be
-Create a bgpvpn:
neutron bgpvpn-create --name rski1 --import-targets 88:88 --export-targets 88:88 --route-distinguishers 11:11
-create net assocs with the two networks:
bgpvpn-net-assoc-create --network net1id bgpvpnid
bgpvpn-net-assoc-create --network net2id bgpvpnid
-try to ping instance 2 from instance one: does not work.

It seems like the mechanism that ensures that layer 2 routing is disabled and instead packets are routed on layer 3 does not work. ARP related flows are active and the packets' dst ethernet address fields are not rewritten.

Tested with Boron 0.5.1-20161017.001225-477