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

@@ -54,20 +54,19 @@ def _rest_definition(self):
 
 class DynamicNAT(BaseNAT):
     def __init__(self, router_id, gateway_interface=None, redundancy=None, mapping_id=None,
-                 mode=asr1k_constants.SNAT_MODE_POOL, bridge_domain=None):
+                 mode=asr1k_constants.SNAT_MODE_POOL):
         super().__init__(router_id, gateway_interface, redundancy, mapping_id)
 
         self.specific_acl = True
         self.mode = mode
 
         self.id = utils.vrf_to_access_list_id(self.router_id)
-        self.bridge_domain = bridge_domain
+        self.bridge_domain = None
+        if self.gateway_interface:
+            self.bridge_domain = self.gateway_interface.bridge_domain
 
     @property
     def _rest_definition(self):
-        if self.gateway_interface is not None:
-            self.bridge_domain = self.gateway_interface.bridge_domain
-
         if self.mode == asr1k_constants.SNAT_MODE_POOL:
             return l3_nat.PoolDynamicNat(id=self.id, vrf=self.router_id, pool=l3_nat.NatPool.gen_id(self.router_id),
                                          bridge_domain=self.bridge_domain, redundancy=self.redundancy,
@@ -78,6 +77,15 @@ 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