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