add an optional parameter to the API that contains a route distinguishers list

The route distinguisher used to announce bgp routes can be auto generated by the bgpspeaker.
However, some use cases need to specify a list of RDs that will be used for route advertisment.

To that aim, the admin should be able to register the list of usable RDs per BGPVPN connection. The backend will then use one those RDs to advertise VPN routes. If no RD list is defined, the backend will autogenerate the RD.