resetting cronjob configuration from manually set schedule

[Tianqi Xiao]

Currently, after customizing cronjob schedule with "set" or "random" keyword in `update-cron-daily-schedule` config option, there's no charm-level operation to bring the value back to default. Changing the config value to "unset" has no effect in this case. This is not the expected behavior.

To resolve this issue, the most obvious way is to implement logic for updating cronjob schedule when the config value is "unset" during config-changed hook. Alternatively, if, for any reason, the previously mentioned method is impossible, we could consider adding an action for resetting the schedule.