Check reserve/unreserve fixed-ips APIs attributes

This patch adds the JSON schema for Nova V2 reserve & unreserve
fixed-ips APIs response and validate the response with added JSON
schema to block the backward incompatibility change in the future.

The response body of V2 reserve & unreserve fixed-ip API is below:

{
    "202 Accepted The request is accepted for processing."
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Ic837fb3cdc34a79c25589a7263f54b96b6043680
Merge "Check reserve/unreserve fixed-ips APIs attributes"