Snapshot Stack

Add Snapshot for stack-snapshot.
We current use Backup for stack-snapshot because snapshot will lock resource which been snapshotted.
But user can't snapshot resources in stack, which means user can only do backup with longer time and bigger disk space then doing snapshot. For user who did schedule backup will require massive disk space. Also Backup lock Stack for a period of time.
IMO, we can introduce snapshot by some limitation to avoid resource locking issues.