The recommended downloads are OSes defined in the recommended-downloads.xml
file. Some of them are already retrieved from os_db.list_downloadable_oses,
so there's no need to duplicate them.

Custom OSes, such as the Free RHEL option, still need to be added.
A custom OS is an OS Media that doesn't have a defined URL, since it will
be handled specially (a login gate).

Fixes #374