Add "Port" to "RouterPort" foreign_keys

This foreign key is present in the DB model definition [1]. The OVO
definition should reflect it.

Trivial-Fix

[1]25d0ad6197/neutron/db/models/l3.py (L29-L33)

Change-Id: Ib47d4ade934e1a631bddc19f4a78c237da822994

neutron/objects/router.py

@@ -135,7 +135,8 @@ class RouterPort(base.NeutronDbObject):
 
     primary_keys = ['router_id', 'port_id']
 
-    foreign_keys = {'Router': {'router_id': 'id'}}
+    foreign_keys = {'Router': {'router_id': 'id'},
+                    'Port': {'port_id': 'id'}}
 
     fields = {
         'router_id': common_types.UUIDField(),