fix s3 ListAllBackups() to list backups from the root directory
fix s3 ListAllBackups() to list backups from the root directory

Signed-off-by: Yujie Zhu <47d832ba08c3a54a3a1fa9d164a44f7dca958fda@gmail.com>

tinyspeck/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess