Fixes #3860 - automated discovery provisioning

Replaces https://github.com/theforeman/foreman_discovery/pull/97
(from above) You seem to pass a Rule in from the controller, but all the code in this method seems to be aimed at a Host (the var `dhost` implies this). This leads to oddities like `Executing discovery rules for host Debian (1)` when I would expect to see `Executing discovery rules for host mac112233445566 (1)`. I don't see how this can work.