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/tempest/api/identity/admin/test_users_negative.py b/tempest/api/identity/admin/test_users_negative.py
index e9e7818..060f24a 100644
--- a/tempest/api/identity/admin/test_users_negative.py
+++ b/tempest/api/identity/admin/test_users_negative.py
@@ -207,7 +207,7 @@
     @attr(type=['negative', 'gate'])
     def test_get_users_request_without_token(self):
         # Request to get list of users without a valid token should fail
-        token = self.client.auth_provider.auth_data[0]
+        token = self.client.auth_provider.get_token()
         self.client.delete_token(token)
         self.assertRaises(exceptions.Unauthorized, self.client.get_users)
         self.client.auth_provider.clear_auth()