bring back oslo_clean

oslo_clean is still needed at this point, removing it was
premature, especially for upgrade testing.

Change-Id: Ic845d835f587923423f83ac698bd825f3fa5dd1f
Merge commit 'refs/changes/75/84475/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch