Use systemd timers instead of cron

[QuLogic]

Is your feature request related to a problem? Please describe.
The droplet agent package requires cron, and installs files in /etc/cron.* but I otherwise have no other need for cron. Since the packaging instructions say systemd is the preferred form of management, this should apply to the maintenance jobs as well. Stuff like this random delay can be specified directly in the timer unit instead of written manually.

Describe the solution you'd like
Provide systemd timer unit files, and change the packaging to not depend on cron.

[QuLogic]

See also digitalocean/do-agent#266

[house-lee]

Thanks for bringing this to our attention @QuLogic ! I'll discuss it with my team and get back here with any updates.

[iosonogio]

I would like to support this proposal as well.