#6335, #6343 - host creation improvements

I'm trying to make this work:

    hammer host create --hostgroup libvirt --compute-resource localhost --name hammer

It nearly does, only http://projects.theforeman.org/issues/6342 is a problem because of Hammer passing in the keys below compute_attributes (#4250 fixed the API issue itself with compute profiles, meaning if you don't pass compute_attributes at all, or it's empty, then you can create a host entirely from a profile).