add region to clean task, fix default key_starts_with

Its seems reasonable that the default of key_starts_with and prefix_to_clean would be the same, since now it will never work - if key_starts_with is timestamp, but the prefix to clean is a datestamp, no files will ever be found like this

Now, each file should get uploaded (defaultly) in a uploads/20141128 folder and when searching for files to clean it will search for objects that start with a datestamp and not a up-to-the-second time stamp.

Also, task was missing a region, so bucket in non default region caused errors.