When a backup is deleted, the delete method uses ListObjects to get a list of
files it needs to delete in s3. Different s3 implementations may return
the object lists in different, even non-deterministic orders, which can
result in the deletion not working because ark tries to delete a non empty folder
before it tries to delete the files in the folder.
Signed-off-by: Bastian Hofmann <bashofmann@gmail.com>