Merge branch 'dnssec-cron-slaveblock' into 'stable-3.1'

DNSSEC-Cronjob should only run on ispconfig master as we are working with datalo…

…g and not accessing things directly

@tbrehm, am I right that `$app->db->datalogUpdate`for a soa on the ispc master does also update the dns_soa table in master database or do I have to do this by hand?
This could probably be a reason for the issue of @jnorell in #1491 ...

See merge request !446