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
diff --git a/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 990cb37..fe4959b 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -398,8 +398,7 @@
 #uri_v3=<None>
 
 # Identity API version to be used for authentication for API
-# tests. Planned to extend to tenant isolation, scenario tests
-# and CLI tests. (string value)
+# tests. (string value)
 #auth_version=v2
 
 # The identity region name to use. Also used as the other