Merge branch 'AssignUserId-variable' into 'stable-3.1'

AssignUserId as variable instead of hardcoded

See merge request ispconfig/ispconfig3!1056

server/conf/apache_ispconfig.conf.master

@@ -132,7 +132,7 @@ Alias /.well-known/acme-challenge /usr/local/ispconfig/interface/acme/.well-know
         Allow from all
     </tmpl_if>
 	<IfModule mpm_itk_module>
-       AssignUserId www-data www-data
+       AssignUserId <tmpl_var name='system_user'> <tmpl_var name='system_group'>
     </IfModule>
 </Directory>