Unlike Legacy routers, DVR Edge Routers have their gateway
interfaces in the SNAT namespace as opposed to the router
namespace. Added a new method to the router_info class,
get_gw_ns_name(), so callers can determine which namespace
the gateway device lives in. This can then be over-ridden
in the DVR Edge router class.
The Prefix Delegation code will also now listen on the
update_router event from the l3-agent and reset the namespace
name if it changes.
(cherry picked from commit