Support for NVP distributed router

Blueprint nvp-distributed-router

This patch adds support for NVP distributed logical routers
adding a simple attribute extension.
The default router type can be controlled used the default_router_type
option in the nvp section of neutron configuration.
In order to ensure backward compatibility, pre-existing routers
will be treated as centralized routers.

Change-Id: Iaab9ffb6071c93990be711ebb56c212230544a7a
Merge "Support for NVP distributed router"