Verify Set/Get/Delete server meta item attributes

This patch adds the JSON schema for Nova V2 & V3 Set/Get/Delete
server meta item APIs response and validate the response with added
JSON schema to block the backward incompatibility change in the future.

The response body of Set/Get server meta item V2 API is below:

{
    "meta": {
        "foo": "%(value)s"
    }
}

The response body of Set/Get server meta item V3 API is below:

{
    "metadata": {
        "foo": "%(value)s"
    }
}

Delete server meta item API does not return any body.
Response code in V2 & V3 is 204.

Partially implements blueprint nova-api-attribute-test

Change-Id: Icf32a97cb844d7ced1f64a726b4142775df0e1d5
Merge "Verify Set/Get/Delete server meta item attributes"