Validate hypervisors_servers Nova V2/V3 API

This patch adds the JSON Schema for response of Nova V2 & V3
hypervisors_servers  API and validate the response with added
JSON Schema to block the backward incompatibility change in the future.

The response body of V2 hypervisors_servers API is below:

{
    "hypervisors": [
        {
            "id": 1,
            "hypervisor_hostname": "fake-mini",
            "servers": []
        }
    ]
}

The response body of V3 hypervisors_servers API is below:

{
    "hypervisor": {
        "id": 1,
        "hypervisor_hostname": "fake-mini",
        "servers": []
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I0456c9f2cd9576edd8ce457ac805fab3ba03c060
Merge "Validate hypervisors_servers Nova V2/V3 API"