Remove apache_mpm from capistrano_app

This was really only needed in capistrano_wordpress_app, so it has
been moved there.
Change default docroot_dir

The apache2 cookbook v3.0.0 includes significantly tighter file
security by default, so we're changing the default docroot_dir to
correspond with our old directory structure.

This can be refactored in the future to allow a custom value.