Update clean_s3.py script to pass over checkpoints not ready for deletion

[mshannon-sil]

If a user would like their checkpoints to remain past the typical 1 month automatic deletion, they should have a way to specify that in the S3 bucket, so that the clearn_s3.py script passes over their checkpoints. This could be a file containing a date either in its filename or in its contents, before which no checkpoints would be deleted.