add functions to 550-bind_dnssec.inc.php (see https://www.howtoforge.com/community/threads/dnssec-problem-expire-too-soon-ispconfig-3-1.74682/#post-351381)

DNSSEC-Cronjob should only run on ispconfig master as we are working with datalog and not accessing things directly

https://www.howtoforge.com/community/threads/quota-warnings-notifications.73747/#post-347405

- added 'certbot' to which command (if letsencrypt is not installed as system package, but certbot is)

improved crojob

Fixed cron to update serial and not sign directly. This now issues a job for the cron server which will then generate the zone and sign it.
cleaner solution AND fixes a possible replication (zone-transfer) issue.