'Lists keys in a bucket under prefix and not containing delimiter
:param bucket_name: the name of the bucket
:type bucket_name: str
:param prefix: a key prefix
:type prefix: str
:param delimiter: the delimiter marks key hierarchy.
:type delimiter: str'

def list_keys(self, bucket_name, prefix='', delimiter=''):

    b = self.get_bucket(bucket_name)
    keylist = list(b.list(prefix=prefix, delimiter=delimiter))
    return ([k.name for k in keylist] if (keylist != []) else None)
