Refactor db_base_plugin_v2 and to remove code duplication

fixes bug 1104379

Introduce CommonDbMixin which includes utility methods
to manipulate model queries.

Change-Id: Ib3602321328cbf945358e0581ecc649e3f69d196
Initial Modular L2 plugin implementation.

The Modular L2 Plugin uses drivers to support separately extensible
sets of network types and of mechanisms for accessing networks of
those types. This is an initial implementation that has been tested
with the openvswitch and linuxbridge agents, and should also work with
the hyperv agent. See quantum/plugins/ml2/README for details.

Implements: blueprint modular-l2
Change-Id: Ia8cae480180f0990af7d5e5e56f29eaeac205e0e
Merge "Initial Modular L2 plugin implementation."
Fix ml2 stack trace when logging.

Change-Id: I8bbf6edd3e526483e7f04e3eeeea571a660235fa
Merge "Refactor db_base_plugin_v2 and to remove code duplication"
Merge "Fix ml2 stack trace when logging."