Closes-Bug: #1353256. Issue : route target list was update without calling set_route_target_list. Hence _pending_field_updates was not set. This resulted in not sending correct json body in update REST API. Fix: Use vnc_api to update the route_target_list on VN. Also clear it when all route targets are removed

Change-Id: I330eb388c33c2c9c9c034edd324aa30ff0c17873