Fix test_version on Python 3.4

Since Python 3.4, argparse has been printing the version on stdout rather than
on stderr as it used to. We should fix this now because:
- this is an issue when packaging oslo-config in Debian Sid, which uses Python
  3.4;
- it will be needed when Openstack switches to 3.4 anyway.

Change-Id: I6a84009acb8621d62465c4cd2573200a07b6b4ab