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.)
@ohadlevy, Thanks for the feedback.  In this case, we should be Ok since we are only deleting the content host (i.e. the foreman host remains unchanged).  What we are trying to avoid is having a case where there is 1 foreman host and N content hosts, which could result from scenarios such as re-provisioning or a subscription-manager clean & register from the host.