Use different variables in the crontab file to help the install script

Using `!!(*= $vhost_dir *)!!` instead of `/data/vhost/!!(*= $vhost *)!!`
makes it easier for the install script to rewrite the crontab for
systems that keep web host data in directories other that /data/vhost
(e.g. /var/www is common).

This is similar change to 20e447e7356ac0387.  Fixes #1376