manage bgpvpn associations as subresources

considering that we see some use cases to add attributes to an association between Neutron resources (port, network, router) and a bgpvpn resource, we should move from the action model that we currently use to manage the association, to a subresource model.
With a subresource model, the association will have its own uuid. CRUD operations will be available on those association objects. We will be able to easily manage attributes of an association with a CRUD API.

We currently don't see any attributes for the network association.
The router association will have a flag to decide if static_routes of the router have to be announced in the BGPVPN.
In the port association context [1] a prefix attribute will define that a static route for this prefix has to be announced in the bgpvpn with the associated port as nexthop.

[1]https://blueprints.launchpad.net/bgpvpn/+spec/port-association