association of neutron routers to a bgpvpn

This blueprint takes care of the implementation of router associated to bgpvpns
as defined in the docs here http://docs.openstack.org/developer/networking-bgpvpn/api.html#router-association.
Associating a router to a bgpvpn will, behind the scenes in the bagpipe service
driver, act like multiple network association for each network that's attached
to the router. After a router is associated any interface that's added or
removed should also result in the appropriate network association action.

Even though in the service driver it acts like network associations no
network_associations are created in the database. A new router_associations
table will be made and a bgpvpn will have a "routers" attribute added next to
"networks".