Add tests for LVM -cow clearing

Ensure that volume_clear for Thick LVM snapshot deletion
references the -cow device, and that it uses the LV instead
of the -cow device for ThinLVM.

These tests were written for a change to stable/grizzly --
but are applicable for general use here as well.

Related-Bug: #1245529
Change-Id: I380ad7832d558d9aa73a701cbf3acc35229e70ab
Merge "Add tests for LVM -cow clearing"