Fix issue with Pre-Grizzly deployment

In pre-Grizzly OpenStack deployment, the `os-services` extension is
not implemented. This cause an exception to be raised while trying to
create an user to perform the tests.

This patch update the behavior: if the extension is not supported, a
warning will be logged and the program will not exit.

Closes-Bug: 1393891

Change-Id: I038af309b073561a876e0c049a2ac8ef24ba23b5
Merge "Fix issue with Pre-Grizzly deployment"