Add randomness to execution time

[mwgamble]

We have a job that needs to run once a day. We have several instances of this job running across multiple containers, and we'd like to spread the jobs out a bit. The jobs are all scheduled to run at (for the sake of an example) 8pm, but they don't actually all need to run at 8pm, they just need to run at around that time. It would be nice if we could configure a window of time (say 5-10 minutes) around which the job could run, so that they wouldn't all fire at once. This would help us avoid hitting some rate limits with a third-party API.

This is basically the RandomizedDelaySec option for systemd timers: https://www.freedesktop.org/software/systemd/man/latest/systemd.timer.html#RandomizedDelaySec=

[kibertoad]

@mwgamble That would be a great feature! Would you be open to send a PR for it?