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.
I guess a more clear version of my question is: if it find the MD5SUM, will it keep looping to check the SHA1SUM and SHA256SUM? If so, wouldn't that overwrite to result stored when it found the MD5SUM?