Migrate cron to systemd timers

[gregharvey]

As we have already done with ce-provision, we need to start moving away from cron as it is obsolete.

While we are doing that, there is an old backlog task to allow delegation of scheduled tasks to another machine. Here is the text from that:

Right now you can "defer" cron to the deploy server, however in cases where the deploy server is dealing with dozens of sites, and if cron frequency is set quite aggressively, this can choke the deploy server entirely.

So we should look at allowing [the systemd timer role] to set a target server to run [timers] from in the first instance, allowing for "workers" that can handle ASG cron load without impeding the deploy server. Bonus points if Lambda processing is an optional extra. 😉