fixes #5184 - extend foreman host to provide association to katello content host

This commit provides for a few small changes:
1. add an association to the foreman host to the katello content host
2. update the foreman defacing to use the data from this association
   versus querying the System table for the details
3. when a content host is registered, if there exists a foreman host
   with the same hostname, associate the two (host & content host)
4. with the relation added in #1, when the foreman host is deleted
   the corresponding content host will be deleted.
refs #5184 - rename 'remove' to 'unregister' for content host ui