Fix neutron-ovn-metadata-agent with policy.yaml

The config.json template for neutron-ovn-metadata-agent uses a
hard-coded policy file name of policy.json. This prevents use of a
policy.yaml file with this service. This patch fixes this.

TrivialFix

Change-Id: Ib96d68f1dc60a0cbb5b79302c1face9c2272946a

ansible/roles/neutron/templates/neutron-ovn-metadata-agent.json.j2

@@ -12,14 +12,13 @@
             "dest": "/etc/neutron/neutron_ovn_metadata_agent.ini",
             "owner": "neutron",
             "perm": "0600"
-        },
-        {
-            "source": "{{ container_config_directory }}/policy.json",
-            "dest": "/etc/neutron/policy.json",
-            "owner": "neutron",
-            "perm": "0600",
-            "optional": true
         }
+        {% if neutron_policy_file is defined %},{
+            "source": "{{ container_config_directory }}/{{ neutron_policy_file }}",
+            "dest": "/etc/neutron/{{ neutron_policy_file }}",
+            "owner": "neutron",
+            "perm": "0600"
+        }{% endif %}
     ],
     "permissions": [
         {