[Firewall] Fix AttributeError while hitting tap to automatically complete the name of network rule of a firewall (Azure#2651)

Author: Jianhui Harold

src/azure-firewall/azext_firewall/_completers.py

@@ -17,7 +17,7 @@ def completer(cmd, prefix, namespace, **kwargs):  # pylint: disable=unused-argum
         try:
             firewall_name = namespace.firewall_name
         except AttributeError:
-            firewall_name = namespace.resource_name
+            firewall_name = namespace.azure_firewall_name
         if namespace.resource_group_name and firewall_name:
             af = client.azure_firewalls.get(namespace.resource_group_name, firewall_name)
             return [r.name for r in getattr(af, prop)]