Initial Modular L2 Mechanism Driver implementation.

Define the Mechanism Driver interface for create/update/delete
operations on networks and ports. For each of these event, the
Mechanism Driver provides one method that is called within the
database transaction of the ml2 plugin method, and one that is called
after the transaction is completed.

Support for mechanism drivers is still a work-in-progress, and the
interface is subject to change in future versions before the release
of Havana. However this initial version should be sufficient to enable
others to start defining their own mechanism drivers.

Change-Id: Ife30215589792ee27df9897d3b2bc04392638266
Implements: blueprint ml2-mechanism-drivers
Fixes: bug #1199977
Fixes: bug #1199978
DocImpact
Merge "Initial Modular L2 Mechanism Driver implementation."