BGP Dynamic Routing: introduce routing peers

This is the first patch to provide BGP dynamic routing functionality in
Neutron.

Basic new entity for BGP is the Routing Peer, to define from Neutron API
the upstream peers to exchange routes.

A new extension is added with basic CRUD methods to expose the routing
peer entity, as well as the migration script and the new service definition.

In this patch, routing peer does not interact with any of other entities of
Neutron.

Partial-Implements: blueprint bgp-dynamic-routing
Change-Id: If55b5940c16c6fdd55cc15567b430b027b59a50b
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