Verify hypervisor uptime attributes of Nova API

This patch adds the JSON Schema for response of Nova V2 & V3
hypervisor uptime APIs ('os-hypervisors/<hyper_id>/uptime')
and validate the response with added JSON Schema to block
the backward incompatibility change in the future.

The response body of V2 & V3 hypervisor uptime APIs is same
& given below:

{
    "hypervisor": {
        "id": %(hypervisor_id)s,
        "hypervisor_hostname": "fake-mini",
        "uptime": " 08:32:11 up 93 days, 18:25, 12 users,
                  load average: 0.20, 0.12, 0.14"
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I163c50e0ad277ef8ed639363d5e53f66e9f9d4e4
Merge "Verify hypervisor uptime attributes of Nova API"