Verify 'list-server-metadata' APIs attributes

This patch verifies the response attributes of 'list-server-metadata'.

The response attributes of v2 and v3 are the same:
{
    "metadata": {
        "foo": "Foo Value"
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Iab1cb1efc0293e8e0aa5124752ec95048b8608bf

tempest/api_schema/compute/servers.py

@@ -12,6 +12,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import copy
+
 get_password = {
     'status_code': [200],
     'response_body': {
@@ -58,3 +60,5 @@ set_server_metadata = {
         'required': ['metadata']
     }
 }
+
+list_server_metadata = copy.deepcopy(set_server_metadata)

tempest/services/compute/json/servers_client.py

@@ -276,6 +276,7 @@ class ServersClientJSON(rest_client.RestClient):
     def list_server_metadata(self, server_id):
         resp, body = self.get("servers/%s/metadata" % str(server_id))
         body = json.loads(body)
+        self.validate_response(common_schema.list_server_metadata, resp, body)
         return resp, body['metadata']
 
     def set_server_metadata(self, server_id, meta, no_metadata_field=False):

tempest/services/compute/v3/json/servers_client.py

@@ -284,6 +284,7 @@ class ServersV3ClientJSON(rest_client.RestClient):
     def list_server_metadata(self, server_id):
         resp, body = self.get("servers/%s/metadata" % str(server_id))
         body = json.loads(body)
+        self.validate_response(common_schema.list_server_metadata, resp, body)
         return resp, body['metadata']
 
     def set_server_metadata(self, server_id, meta, no_metadata_field=False):