Retutn OS family in lower case

Return OS family name like ohai style.

Now only redhat and fedora are returned correctly as lower case.
I will try to fix others later.
Remove command classes other than base, linux and redhat temporary
Fix according to the change of command class name
Refactor base command class (in progress)
Refactor linux command class (in progress)
Refactor redhat command class (in progress)
Rename detect_os to os
Get Base command class if OS command class does not have the class
related to the target
No more need to require detect_os and include Specinfra::Helper::OS
tiny fix
Add package command class (in progress)
Add check_installed method to redhat/base/package
Move check_os method from backend/exec to helper/os
Move existence class override code to specinfra.rb
Refactor OS detection completely
Fix how to get hostname and port
Fix how to get hostname and port
Specinfra::Command::Base detect os in itself now
Fix os related spec, class name, cache key and so on
Fix spec