Changes to address the changed cache_root and module name in Apache 2.4.

A new mod_name variable is included in apache::mod::disk_cache as it changed from mod_disk_cache to mod_cache_disk. This is used to load the correct module and by the disk_cache.conf template.

The default cache_root changed from /var/cache/mod_proxy to /var/cache/httpd/proxy (on Redhat).

TODO: confirm if the default cache root changed on either Debian or FreeBSD.
@dteirney yeah, you definitely want to use `versioncmp($::apache::apache_version, '2.4') >= 0`