Proxy ARP sometimes floods packets out the port they came in

When ONOS gets an ARP request/reply and the destination host isn't known, the packet is flooded out all known edge ports. This means it gets re-sent out the port that it came in on.

Steps to reproduce:
1. Start an ONOS cluster in a clean state (no devices, links or hosts).
2. Connect a simple Mininet network (such as a linear topology with 5 switches)
3. Pick a target host and dump packets on the host's interface
4. Ping from the target host to any other host. In the packet dump you will see the host send an ARP request, then ONOS will duplicate the same ARP request packet out the port it came in on. The packet dump will show the same packet twice close together on the host's interface.