Validate of Nova list Floating IPs attributes

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

The response body of list Floating IPs API is below:

{
    "floating_ips": [
        {
            "id": 1,
            "pool": "nova",
            "instance_id": null,
            "ip": "10.10.10.1",
            "fixed_ip": null,
        }
    ]
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Ica266688af310bb804bcf7fb240ca6a005b156c0
Merge "Validate of Nova list Floating IPs attributes"