First part of fix for issue #3896 - Unknown column 'default_mailserver' in 'field list' when saving client limits main template

This reverts commit 008ef889.

Removed ubuntu1604.lib.php which is not required anymore.

- Introduced confid in $distver array to separate the config and class for a distribution in the installer.
- Cleaned up the dist detection.

https://mozilla.github.io/server-side-tls/ssl-config-generator/

Changed behaviour of UFW installation. Do not enable it by deafult as the firewall will get enabled autmatically when a firewall record gets added in ispconfig. Enabling it without that record, the Firewall would lock out the user on next reboot.