BGP Dynamic Routing: routing peer's associations

Dynamic Routing basic goal is to add a easy way to advertise routes as
well as learn remote routes to be used by Neutron's routers.

This commit adds the database entities and the API expose functionality
to let define the advertise routes and the binding between the routing
peer and the Neutron router.

Change-Id: If67698b3bbb9d909e5c365f00bda667062ed1264
Partial-Implements: blueprint bgp-dynamic-routing