ofagent: Enable local arp responder for local VMs

Because ofagent makes every arp requests packet-in and
perform packet-out anyway, just make it respond arp requests
for local VMs.  It would be simpler and more effective than
forwarding requests to the network as it did before this change.

Before this change:
    request: src VM -> switch -> controller -> switch -> target VM
    reply: target VM -> switch -> src VM

After this change:
    request: src VM -> switch -> controller
    reply: controller -> switch -> src VM

As a bonus, now we can test l2pop and local arp responder with
a single node setup, at least for some extents.

Related: blueprint ofagent-l2pop
Related: blueprint ofagent-merge-bridges
Change-Id: I606a238d8c2021ed908fbd89d3488768497f57c5
Merge "ofagent: Enable local arp responder for local VMs"