Implement ML2 port binding

Implement ML2 port binding

The ml2 plugin uses mechanism drivers to determine which network
segment and what VIF driver to use for a port. Mechanism drivers
supporting the openvswitch, linuxbridge, and hyperv agents are
added. The binding:host attribute is set on ports belonging to the
dhcp and l3 agents so that they can be bound.

To use with devstack until it is updated, set
"Q_ML2_PLUGIN_MECHANISM_DRIVERS=openvswitch,linuxbridge" in localrc.

The hyperv L2 agent does not currently implement the agents_db RPC,
and will therefore not work with its ml2 mechanism driver. This issue
will be tracked as a bug to be fixed in a separate merge.

implements blueprint: ml2-portbinding

Change-Id: Icb9c70d8b0d7fcb34b57adc760bb713b740e5dad