Configurable external gateway modes

Blueprint l3-ext-gw-modes

This patch introduces an API extension for enabling or disabling
source/destination NAT on the router gateway interface, and implements
support for this extension in the l3 agent, which has also been
refactored in order to ensure SNAT rules are applied (or removed)
in a single place in the process_router routine.
The patch therefore enables the extension on all plugins which
leverage this agent.
In this patch the validate_boolean function is re-introduced, as it
used for validating a field of the external_gateway_info dict.
The resource extension process is also slightly changed so that an
API extension can specify which extension should be processed before
its attributes can be processed. This is needed to ensure the
proper validator for external_gateway_info is installed.

Change-Id: Ia80cb56135229366b1706dd3c6d366e40cde1500
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 "Configurable external gateway modes"
Merge "Initial Modular L2 plugin implementation."
Merge "Refactor db_base_plugin_v2 and to remove code duplication"