Decouple ARP responder from Neutron subnet information

Dear Netvirt team

Starting in Boron, the L3VPN service requires Neutron networks and subnets to be configured in order to work. This is a change in behavior from Beryllium where the L3VPN API was self-sufficient to configure a working L3VPN.

The underlying technical reason for this change is that the ARP responder implementation relies on Neutron subnet information. Specifically, the ARP responder creates flow entries for the gateways defined in the Neutron subnets. In Beryllium, the ARP service did not require this but responded to ARP requests independently of subnet gateways. The L3VPN service was subsequently able to route packets to the corresponding destination.

This bug report requests a decoupling of the ARP responder implementation from Neutron subnet information - corresponding to the previous behavior.

Best regards
Georg