better stubbing of distro

- only 1 'local' method Distros.local
- don't 'exist' on a release_file that doesn't exist
- consistent stubbing of distro.
move distro constants into instance variables

- move package distro case statement into distro itself
- remove redhat distro constant (it is rhel or fedora)
- leverage Distros.all instead of meta class fu
Clean up etc_issue

- remove knowledge of internals from specs
- lazy load file
- add include? method
use a PATH instead of hardcoding commands
remove extra distro classes
Merge pull request #96 from kbrock/use_path

Use path instead of individually hard coded commands