Check create/get/delete V2 server_group attributes

This patch adds the JSON schema for Nova V2 create, get & delete
server_group APIs response and validate the response with added
JSON schema to block the backward incompatibility change in the future.

The response body of create & get server_group V2 APIs is same and given
below:

{
    "server_group": {
        "id": "5bbcc3c4-1da2-4437-a48a-66f15b1b13f9",
        "name": "test",
        "policies": ["anti-affinity"],
        "members": [],
        "metadata": {}
    }
}

Delete server_group API only return 204 response code.

Partially implements blueprint nova-api-attribute-test

Change-Id: I0447292553394877493fc264451559d9df14242f
Merge "Check create/get/delete V2 server_group attributes"