ofagent: Local arp responder for VLAN

Implement local arp responder for non-tunnel network types.
Enable it for VLAN by requesting l2pop for VLAN.

Related: blueprint ofagent-l2pop
Related: blueprint ofagent-merge-bridges
Change-Id: I2f200a1ae2bd769ea1f83fe65e58e82325ed92b0
Merge "ofagent: Local arp responder for VLAN"