fixes #7005 / BZ 1122736 - content host - change registration behavior with foreman host

This commit alters the behavior of registering a content host
(with or without an activation key) that is to be associated
with a foreman host to do the following:

1. identify the foreman host that this content host will be
   associated with by both name and mac address (previously
   it was only by name)
2. if the foreman host identified is already associated with a
   content host, delete it and then create/associate a new one.
   (This will avoid there becoming duplicate content hosts with
   the same name.)
Maybe I'm missing the point, but just FYI when deleting a foreman host a few things can happen including deleting the VM itself.. Handle with care on when you actually delete it.