Make apache_mods_enabled work.

The variable apache_mods_enabled was always overridden by
vars/Debian.yml.  It should be placed in defaults/main.yml
instead so the user's changes take effect as specied in the
README.

Also deleted superfluous files defaults/Debian.yml,
defaults/RedHat.yml and vars/main.yml.  The former two were
never read and the contents of the latter one should be placed
in defaults/main.yml.