Hard delete backup stack after successful update.

After successful update of a stack, the backup stack and it's associated
template, resources and events should be permanently deleted.

Closes-Bug: #1332300

Change-Id: I3a462690b5f6bfadfbcf16eeb6bdec7c5ece2e83
Add current_size attribute for OS::Heat::AutoScalingGroup

User can use this output attribute to get the current size
of AutoscalingResourceGroup.

Change-Id: I6756e85aa3435a822aa72139cac7aabd8254079d
Merge "Add current_size attribute for OS::Heat::AutoScalingGroup"
Merge "Hard delete backup stack after successful update."