backend: Run aptly cleanup script on a weekly basis

This aptly script performs a database cleanup to remove information
about unreferenced packages. Also deletes files in the package pool that
aren't used by packages anymore.

Aptly documentation suggests that it's a good idea to run this command
after massive deletion of mirrors, snapshots or local repos.

This commit adds the script to be called in a weekly basis by cron.
Note that with this configuration it will be called each Sunday at
00:00 hours.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>