onionoo-backend: replace @reboot cronjobs with systemd service units

Currently puppet installs two cronjobs on onionoo-backend servers that are started only at reboot. This causes issues with needsrestart since the jobs (when running with deprecated libraries/binaries) can't be restarted. If one of the services crash there's also no means to restart it. The only way out of the two issues is to reboot the hosts.

To remedy that situation, we should be using systemd service unit files instead of an @reboot cronjob. The systemd unit can be created system-wide and using user/group options to drop privileges.