Check hypervisor statistics attributes of Nova API

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

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

{
    "hypervisor_statistics": {
        "count": 1,
        "current_workload": 0,
        "disk_available_least": 0,
        "free_disk_gb": 1028,
        "free_ram_mb": 7680,
        "local_gb": 1028,
        "local_gb_used": 0,
        "memory_mb": 8192,
        "memory_mb_used": 512,
        "running_vms": 0,
        "vcpus": 1,
        "vcpus_used": 0
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I7cd1e81c722e0117b9f2fc98e9b2d2bf7eb5177c
Merge branch 'master' of https://github.com/openstack/tempest