Configurable external gateway modes

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