Implement local ARP responder onto OVS agent

Implement local ARP responder onto OVS agent

With ML2 plugin and l2-pop mechanism driver, it's possible to locally
answer to the ARP request of the VM and avoid ARP broadcasting emulation
on overlay which is costly.

When this functionality is enabled, the OVS flows logic evolves to [1].
This functionality was introduce in 2.1 OVS branch [2].

A README is added to describe l2-pop mechanism driver and the agents
particularities.

[1] https://wiki.openstack.org/wiki/Ovs-flow-logic#OVS_flows_logic_with_local_ARP_responder
[2] http://git.openvswitch.org/cgi-bin/gitweb.cgi?p=openvswitch;a=commitdiff;h=f6c8a6b163af343c66aea54953553d84863835f7

DocImpact: New OVS agent flag 'arp_responder' set to false by default
Closes-Bug: #1237427
Change-Id: Ic28610faf2df6566d8d876fcd7aed333647970e2