copy install/tpl/apache_ispconfig.conf.master to server/conf/apache_ispconfig.conf.master

If you add/remove server IPs the file `/etc/apache2/sites-available/ispconfig.conf` gets recreated and it is missing the alias for Let's Encrypt ACME challenges. Copying over the current install template fixes that. 

Maybe one could remove this template duplication completely by using `server/conf/apache_ispconfig.conf.master` in the install process?

The `Header set Content-Type "text/plain"` was only in `server/conf/apache_ispconfig.conf.master` and not in `install/tpl/apache_ispconfig.conf.master` – is it necessary?