Multiversion authentication part2

Address last comments from part1:
- replace auth_data[0] with get_token()
- add token property to rest_client
- Remove comment about planned changes from config.py

Partially implements: bp multi-keystone-api-version-tests

Change-Id: Ia54cd3d824e44dd99d9226d4c0c6d6861d8c12ae
Merge "Multiversion authentication part2"