Add l2 population base classes

This patch initiates the blueprint l2-population
Implemented as a ml2 Mechanism driver.

OVS & LinuxBridge drivers will be added as dependencies.

Rebased on ML2 Portbinding.

Change-Id: Ia2345aa262ec791c9f38b6e41e1e4b46f69cadac
OVS agent implementation of l2-population

This patchset implements l2-population RPC callbacks in OVS agents,
it enables plugin to populate forwarding table following portbindings
events.

For now, it doesn't include ARP responder implementation which is
deferred to a future patchset (As this feature isn't yet supported by
OVS, it will require the use of an external responder such as ebtables)

It anyway brings some improvements in tunnelling management, as agent
will tear-down unecessary tunnels, and flood packets on a per-network
basis rather than to all other agents.

These changes should anyway have a limited risk, as tunnel management
won't be affected as long as l2_population option is not set. This
option must be used in conjonction with ml2 plugin using l2population
mechanism driver.

Implements: blueprint l2-population

Change-Id: I5185eefedb0ff392bc8b99d16f810813e26ff58d