No dynamic NAT for routers without external gw

Routers that do not have an external gateway also don't have a bridge
domain associated with it, meaning the dynamic NAT list would always
produce a diff. Therefore we delete the dynamic NAT list if there's
nothing to configure and also make sure it doesn't appear in the diff.

asr1k_neutron_l3/models/neutron/l3/nat.py

@@ -78,6 +78,15 @@ def _rest_definition(self):
                                               bridge_domain=self.bridge_domain, redundancy=self.redundancy,
                                               mapping_id=self.mapping_id, overload=True)
 
+    def diff(self, should_be_none=False):
+        return super().diff(should_be_none=not bool(self.bridge_domain))
+
+    def update(self):
+        if self.bridge_domain:
+            return super().update()
+        else:
+            return super().delete()
+
 
 class NatList(BaseNAT):
     yang_model = None