Validate for list flavor attributes of V2/V3 APIs

This patch adds the JSON schema for Nova V2/V3 list flavor APIs
response and validate the response with added JSON schema to
block the backward incompatibility change in the future.

The response body of V2 & V3 list flavor APIs is same and given below:

{
    "flavors": [
        {
            "name": "m1.tiny",
            "links": [
                {
                    "href": "http://openstack.example.com/v2/flavors/1",
                    "rel": "self"
                },
                {
                    "href": "http://openstack.example.com/flavors/1",
                    "rel": "bookmark"
                }
            ],
            "id": "1"
         }
    ]
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Ie0539e2b81e6a3446cbd705afe223c8a178bd13f
Merge "Validate for list flavor attributes of V2/V3 APIs"