fixes stack deletion failure, when vol in deleting

when an delete operation is requested on a stack, and the volume in stack
is already in 'deleting' state, heat fails to delete the stack and goes to
DELETE_FAILED state instantly.

Instead of going to DELETE_FAILED state instantly, heat needs to safely
wait for the volume to complete its deletion process, so that heat can
then go ahead and complete stack deletion successfully.

Closes-Bug: #1362708

Change-Id: I088eab90bee4bc804abe9e505fd61b17b266def1
Merge "fixes stack deletion failure, when vol in deleting"