Enable zero the snapshot when delete snapshot in LVMVolumeDriver

Because snapshot without 'size' field, So clear_volume method in
LVMVolumeDriver will skip secure deleting. Get the size of snapshot from
'volume_size' filed, So it can zero the snapshot.

Remove the 'size_in_g' parameter in _delete_volume method, because it never
used. Add a unittest for clear_volume method.

Fixes  Bug #1198185

Change-Id: Ie919b50ce4fb276f29ab2e0279f868a691ea7bef
Merge "Enable zero the snapshot when delete snapshot in LVMVolumeDriver"