Fixes #3860 - automated discovery provisioning

Replaces https://github.com/theforeman/foreman_discovery/pull/97
> This error meesage is misleading when the state is set but the Setting is false.

For all the related comments - yeah, I fixed the if clause to give
proper error message. When it's disabled, it's disabled and we don't
actually do anything.

-- 
Later,
 Lukas #lzap Zapletal