Add iso_download task

The task accept both file and directory URLs for download an ISO image.
By specifying a directory the task will look for SUM files to fetch the
ISO filename, this is useful to grab the latest ISO image.
if there were three ISOs in there, one for each of those values ('MD5SUM', 'SHA1SUM', 'SHA256SUM'), would this keep rewriting the result until the eventually the SHA256SUM is left?