Verify V2 list server_group API attributes

This patch adds the JSON schema for Nova V2 list
server_group API response and validate the response with added
JSON schema to block the backward incompatibility change in the future.

The response body of List server_group V2 API is given below:

{
    "server_groups": [
        {
            "id": "616fb98f-46ca-475e-917e-2563e5a8cd19",
            "name": "test",
            "policies": ["anti-affinity"],
            "members": [],
            "metadata": {}
        }
    ]
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I554237d9b66b42ea75ef3b47c8a9038c80b3cfb5
diff --git a/tempest/services/compute/json/servers_client.py b/tempest/services/compute/json/servers_client.py
index 23c1e64..9c26ac9 100644
--- a/tempest/services/compute/json/servers_client.py
+++ b/tempest/services/compute/json/servers_client.py
@@ -523,6 +523,7 @@
         """List the server-groups."""
         resp, body = self.get("os-server-groups")
         body = json.loads(body)
+        self.validate_response(schema.list_server_groups, resp, body)
         return resp, body['server_groups']
 
     def get_server_group(self, server_group_id):