L2 Agent-side additions to support DVR

This patch introduces changes to the L2 agent, whereby the L2 agent
relies on a DVR component that takes care of the port wiring and the
management of tunnels in face of topology changes due to the life
cycles or VM's as well as the life cycles of distributed virtual
routers.

Support for DVR needs to be explicitly enabled. Default behavior
remains unchanged.

Partially-implements: blueprint neutron-ovs-dvr

Change-Id: If75225898a6f0aeea8b0300b711ca7e01f6b4f9a