Early deprecation of Host::Base#model_name

The #model_name= setter on Host is now deprecated, please use
#hardware_model_name= instead. It conflicts with Rails 4.2+ so I've
renamed it in preparation, and it'll be removed in Foreman 1.12 (one
release earlier than usual).

The association will remain on model_id and the API is also unchanged,
this only affects direct Host usage via the *_name accessor.

More info under ticket #13440.

··· -- Dominic Cleal dominic@cleal.org