Verify service attributes through Nova "get services" API

This patch adds checks whether a response of Nova "get services" API
includes the attributes to block the backward incompatibility change
in the future.

This patch implements the base part of a response validation.
The design is the following:
 * Each API schema is defined under tempest/api/compute/api_schema/
 * If API schemas of v2 and v3 are the same, define common API schema
   under tempest/api/compute/api_schema/
 * Otherwise, API schemas of v2 should be defined under v2/ and the
   one of v3 should be under v3/
 * Each API schema defines the succeeded status code('status_code')
   and response body('response_body')

Partially implements blueprint nova-api-attribute-test

Change-Id: Id0b4c31d47f7c6abafcb3c2ded9309fac61cb3dc
Merge "Verify service attributes through Nova "get services" API"