Please provide a systemd timer as alternative to the crontab entry

On a Fedora installation, `do-agent` and `droplet-agent` are the only two things that pull in `cronie` and `cronie-anacron`. On systems where you're providing a systemd service anyway, it'd be nice to replace the crontab entry by a systemd timer and drop the dependency on `cronie`.

Something like this should work:
```ini
# do-agent-update.timer
[Unit]
Description=…
Wants=network-online.target

[Timer]
OnCalendar=*-*-* 01:00
RandomizedDelaySec=900
Persistent=true

[Install]
WantedBy=timers.target

# do-agent-update.service
[Unit]
Description=…
After=network-online.target

[Service]
Type=oneshot
Nice=19
IOSchedulingClass=2
IOSchedulingPriority=7
ExecStart=/opt/digitalocean/do-agent/scripts/update.sh
```
See e.g. `dnf-automatic.timer` and `dnf-automatic.service` for an example of how to do this well.