stroageTests.py: Call detachStorageDomain on rollback

vdsClient.deactivateStorageDomain() should be called as
vdsClient.activateStorageDomain() 's rollback:

This was missing from the original flow.
activateSD created the mounts, while deactivateSD unmounts the
directories used for the SDs. Not calling deactivate leaves the mounts
lingering in /rhev/data-center. This patch removes these lingering
mounts. The test passes either way.

Change-Id: I943c117b36ac3dd6ccba1e195853b53d83df43c8
Signed-off-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23725
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>