Logrotate for systemd-journald

We currently let the systemd journal take up as much space as it needs on the root filesystem of our machines.

We also ship those logs to banco (using journalbeat), and we make sure that works using a nagios check (/usr/lib/nagios/plugins/swh/check_journal).

We should add a cron that makes sure that the logs have been shipped, and does the rotation using journalctl --vacuum-time.

---

Migrated from T789 (view on Phabricator)