Verify list agents attributes of V2/V3 APIs

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

The response body of v2 and v3 API is the below:

{
    "agents": [
        {
            "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: I59fc1a54b32554d7c9827ce912d3197f78c4cd12
Merge "Verify list agents attributes of V2/V3 APIs"