Improve document root fix for Red Hat (#241)

The docroot fix from PR#240 worked fine for default servers.
But as soon as one uses vhosts this broke as the vhosts were
created under the docroot in /var/www/html rather than the better
/var/www.

Fix this by differentiating between docroot and wwwdir.
Further allow to override both in Red Hat-style configs.
The override allows to reuse this formula for the softwarecollections
rpms.