Verify tenant usages API response attributes

Now most attributes of Nova APIs are not checked in Tempest,
and this patch adds some tests which check these attributes to block
the backward incompatibility change in the future.

This patch adds the checks of tenant usages API responses.
The response body of this API is the following:

{
    'tenant_usages': [{
        'server_usages': [{
            'ended_at': '2014-03-23T23:09:03.000000',
            'flavor': 'm1.nano',
            'hours': 0.0033333333333333335,
            'instance_id': '64093cb4-9185-4b13-b375-53124bd70728',
            'local_gb': 0,
            'memory_mb': 64,
            'name': 'server-412020127'
            'started_at': '2014-03-23T23:08:51.000000',
            'state': 'terminated',
            'tenant_id': '9461ce3ff1a94bdfa5a3ffdecfd5fa3a',
            'uptime': 12,
            'vcpus': 1,
        }]
        'start': '2014-03-22T23:09:40.241602',
        'stop': '2014-03-23T23:09:40.652279',
        'tenant_id': '9461ce3ff1a94bdfa5a3ffdecfd5fa3a',
        'total_hours': 0.0033333333333333335,
        'total_local_gb_usage': 0.0,
        'total_memory_mb_usage': 0.21333333333333335,
        'total_vcpus_usage': 0.0033333333333333335,
    }]
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Ia4be5e5c0175d9ba524a4e97129b6cd0f8125ad2
Merge "Verify tenant usages API response attributes"