Fixes #3860 - automated discovery provisioning

Replaces https://github.com/theforeman/foreman_discovery/pull/97
> I guess we should rename this to "Discovery Manager" to match the other full-edit roles in core?

Sure, but is it worth the effort? I mean, I can't rename it here just
like that. We need a DB SQL migration I guess.

-- 
Later,
 Lukas #lzap Zapletal