Don't zero out snapshot volume on snapshot_delete

Don't zero out snapshot volume on snapshot_delete

 When trying to zero out an LVM snapshot on precise the kernel sometimes
 hangs when performing the dd.  Also the dd process itself can take
 an extremely long time even when it does succesfully complete.
 This can be up to 30 minutes for a 1 Gig volume/snapshot.

 I believe this is a kernel specific issue with LVM snapshots.  The
 zeroing process is unreliable and can cause kernel hangs to let's remove it.

 In order to protect against data leakage we'll implement the zeroing process
 on volume creation.  This doesn't seem to have a significant impact and doesn't
 suffer from the same isues tha zeroing out an LVM snapshot does.

 No reason to continue zero on delete, the zero on creation should
 probably be sufficient.

 Doesn't seem to cause any timing issues but need to keep this in mind.

 Fixes bug 1023755

Change-Id: I56209d8e5973cffa997b4ec3e51c3361838386de