cron: better behavior of crontabs tracking

Nowodays, crontab are pretty painful to manage as whenever we need to change a cron state, we need to track and manage the absenteism of old crontabs. Without, we will clutter again and again the user crontab with new crontabs without purging old ones.

This pull request adds an optionnal parameter 'identifier' which let then us have the two behaviors, the old one and the new which track thanks to identifier the crontabs lines and replace them with the new versions without any effort !