Clear correct device when deleting a ThinLVM snap

The LVM driver tries to wipe a device using dd when
a snapshot is deleted.  For ThinLVM, it is attempting
to wipe the /dev/mapper/cinder--volumes-<vol>-cow device,
which does not exist for ThinLVM.  This causes snapshot
deletion to fail for ThinLVM if volume clearing is on.

When using ThinLVM, only wipe the LV device rather than
attempting to wipe the -cow device.

This bug is only present in stable/grizzly (2013.1.4).
It does not exist in stable/havana or master due to these
changes:
f702fe7 Fix secure delete for thick LVM snapshots
de43923 Don't zero out thin provisioned LV's on delete

Closes-Bug: #1245529
Change-Id: I1d8dd618e51cc99b8947fbe0697a1c765c8251ba