Comment 0 for bug 2017798

`update-cron-daily-schedule` config modifies entries in sudo user crontab (which will be saved to `/var/spool/cron/crontabs/root`) instead of system crontab file (`/etc/crontab`), leaving the latter intact. This has two consequences:

1. both cron configs will be used to run cronjobs, so the scripts will be ran twice
2. `hookenv.log` cannot be correctly executed when running with sudo user cronjob so its messages will not show up in juju debug-log.

We should instead only manipulate `/etc/crontab` file, either by manually changing the file entries, or using dedicated library to achieve so.