Verify the response attributes of 'aggregate-set-metadata' API

This patch verifies the response attributes of 'aggregate-set-metadata'.
The response attributes of v2 and v3 are the same:
{
    "aggregate": {
        "availability_zone": "nova",
        "created_at": "2012-11-16T06:22:22.342791",
        "deleted": false,
        "deleted_at": null,
        "hosts": [],
        "id": 1,
        "metadata": {
            "availability_zone": "nova",
            "key": "value"
        },
        "name": "name",
        "updated_at": null
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I8940a18e71540aec300023e6736a3318b253d702
Merge " Verify the response attributes of 'aggregate-set-metadata' API"