ofagent: Enable local arp responder for TYPE_FLAT

handling TYPE_FLAT is particularly useful because,
as stated in README, a flat provider network is
the recommended way to provide public network connectivity
for ofagent.

Related: blueprint ofagent-l2pop
Related: blueprint ofagent-merge-bridges
Change-Id: I4f9145ecb6ade44fc884ce7f8842f5e33385391e
Merge "ofagent: Enable local arp responder for TYPE_FLAT"