mod_disk_cache doesn't belong in apache::default_mods for redhat

While the module does get loaded on a stock redhat-family apache install, it's actually not enabled; the config is commented out in the httpd.conf provided by the httpd rpm.

However, the apache::mod::disk_cache class both loads the module *and* turns it on. This would undoubtedly cause some unexpected behavior for a number of sites, especially considering $apache::default_mods defaults to true. If the goal of apache::default_mods is to try and mimic a default setup for each OS flavor, then this mod should be removed from this class.

This may also be true for a handful of other apache mods.