Please add config option(s) to be able to randomise cron.d timings within a certain window

[Barry Price]

Currently the charm allows us to either configure an exact time for the log-archive job to run (on every machine in the given service/application), or a randomised time.

This is useful for staggering log-archive runs to avoid saturating the connection, but since it's truly random, there's no way to avoid certain times of day to avoid clashing with other e.g. nightly backups or other bandwidth-intensive processes.

I'm thinking of something like this, but perhaps there are more efficient methods:

  cron_blacklist_begin:
    default: ""
    description: Start of a daily window during which randomised log-archive jobs may not run
    type: string
    value: "06:00"
  cron_blacklist_end:
    default: ""
    description: End of a daily window during which randomised log-archive jobs may not run
    type: string
    value: "09:00"
  cron_time:
    default: true
    description: When to run the log-archiver (crontab(5) format timespec). Use <<rnd>>
      for a random value, cf. cron_blacklist_begin and cron_blacklist_end
    type: string
    value: <<rnd>> <<rnd>> * * *