OFAgent: Implement arp responder

This is step 2 implementation of OFAgent l2-population.
This is handling an arp packet which received by a packet-in
Build and send an arp-reply packet if a packet is an arp.

Change-Id: Ida714f30c0f02c54dda3402c0dbf6047bc182b22
Partially implements: blueprint ofagent-l2pop