Validate get fixed-ips attributes of Nova V2 API

This patch adds the JSON schema for Nova V2 get fixed-ips API
response and validate the response with added JSON schema to
block the backward incompatibility change in the future.

The response body of V2 get fixed-ip API is below:

{
    "fixed_ip": {
        "address": "192.168.1.1",
        "cidr": "192.168.1.0/24",
        "host": "host",
        "hostname": "openstack"
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I1b8b2c00492e85e6075c036b41f28e68357da653
Merge "Validate get fixed-ips attributes of Nova V2 API"