Check if snapshot is deleted cleanly.

This patch added a check in delete_snapshot to make sure the
snapshot is deleted cleanly.  Without this check, if someone
tries to delete the source volume immediately after the snapshot
deletion, it will fail with error that the volume still has
snapshot.

Closes-Bug: #1286699

Change-Id: Idf4508025b403fa572584e5204b3891375fed6c6
Merge "Check if snapshot is deleted cleanly."