Verify create agent attributes of V2/V3 APIs

This patch adds the JSON schema for Nova V2/V3 create agent APIs
includes the attributes to block the backward incompatibility change
in the future.

The response body of v2 and v3 API is the below:
{
    "agent": {
        "agent_id": 1,
        "hypervisor": "xen",
        "os": "linux",
        "architecture": "x86",
        "version": "7.0",
        "url": "xxx://xxx/xxx/xxx1",
        "md5hash": "add6bb58e139be103324d04d82d8f546"
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I86aeae6a6337ce60e2b69e207a6e015950b95a36
Merge branch 'master' of https://github.com/openstack/tempest