In revert_to_snapshot method, snapshot param must be a object and
also has 'id' attribute, so before use the method, we have to use
snapshot.get method to get snapshot information.
But revert_to_snaoshot only use id of snapshot, we can use snapshot
instead of snapshot.id.
Change-Id: Ifbdbae3ee66d72f9d34cf4a8fdf2bde388b2b6f0