[WIP] fixes #6609 - katello templates for image-based hosts

Don't review it yet.  Also requires https://github.com/theforeman/foreman/pull/1586.
I've moved this because Foreman, unfortunately, has two paths for rendering - unattended via the controller, and the image-based ones.  For the image-based ones, everything operates in the context of Host, and it doesn't have access to the overridden SAM url in the KatelloUrls helper.