Enable nova-metadata health check

This patch enables healthcheck execution for nova-metadata container.

Change-Id: I8fe646a569be6c6c6fbd983546af3a1181bc370b

docker/services/nova-metadata.yaml

@@ -104,6 +104,8 @@ outputs:
             user: nova
             privileged: true
             restart: always
+            healthcheck:
+              test: /openstack/healthcheck
             volumes:
               list_concat:
                 - {get_attr: [ContainersCommon, volumes]}