Changing logrotate-cronjob-frequency config adds new frequency instead of replacing the old one

[Tianqi Xiao]

The currently implementation doesn't delete the script file from the previous cron folder when `logrotate-cronjob-frequency` gets updated. Here is an example workflow:

1. set `logrotate-cronjob-frequency` to `hourly`
2. charm-logrorated script gets added to `/etc/cron.hourly`
3. set `logrotate-cronjob-frequency` to `daily`
4. charm-logrorated script gets added to `/etc/cron.daily` but the script in `/etc/cron.hourly` also remains.

The result of this example workflow is running the cronjob in both hourly and daily frequency, which is not the expected behavior.