Update letsencrypt.inc.php as per the discussion at ispconfig/ispconfig3#5030. Already pr'ed and merged in master via ispconfig/ispconfig3!778. Perhaps can be merged with stable too?

Add description text for logging options.

Fixed#5019 LE cert problem in mirror setups. This patch disables the creation of Let's encrypt SSL certs on mirrored slave servers.

Added a third parameter to $app->log() function in server part to turn off logging to database and use this option in mysql lib to not log any server side SQL errors to the database.

- Fixed #5027 add support for PHP 7.1 and 7.2 on nginx servers in apps vhost.
- suppress some warnings in simplepie.inc.php

global option to enable pagespeed with nginx - allows to enable pagespeed even if no webserver-config is selected

- fixed backups not downloadable on multiserver slave, fixes #4973

- fixed accidently deleting mysql r/o user completely when it is removed from a single database, fixes #4963

Fix bug where in a multiserver setup, deleting a backup will not remove its details from the master database as well.

Fix typo in parent user related variable when creating new jailkit shell user. This bug was interfering with the correct functioning of the plugin.

every cron execution. The logfile can be disabled by using -NoLog