To enable or disable a certain module, use the webserver::module::enable() and webserver::module::disable() types available in the module. These types figure out whether the module to enable or disable is a stock module, a package module, or a webapplication module.

mod_alias for example, which enables the use of Alias /path /to, is a stock module, since it ships with the httpd package.

mod_python however is a package module, as it does not ship with the httpd by default, and requires you to install the mod_python package. However, different from the webapplication modules, this package module makes available an actual module to the httpd application.

mediawiki however is a webapplication and does not install any modules used by httpd, but does require some stock modules and package modules to be installed.

node 'node1.example.org' {
    include webserver

    webserver::module::enable { "mod_python":
    }
}