AWS: Ensure that the order returned by ListObjects is consistent
AWS: Ensure that the order returned by ListObjects is consistent

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 <06a847907a915832fbf09d2fe7ae428187442792@gmail.com>

heptio/ark,heptio/ark