Correct broken mime_magic config for Debian

The location "conf/magic" in the mime_magic.conf template is RedHat specific
and is broken under Debian/Ubuntu. Added a class parameter with an os agnostic
default and matching template usage.

Added full testing and updated `README.md` to indicated this module has
settings and template parameters.
Merge pull request #421 from dbeckham/mime_magic_ubuntu_fix_squashed

Correct broken mime_magic config for Debian - Squashed commit for #418