Fixes #10111 - Use a dummy primary interface for unmanaged hosts

Unmanaged hosts require a host interface due to delegation of networking
attributes, and some others like 'name'. Since unmanaged hosts do not
necessarily have an interface associated (unless created through puppet
facts importer), we provide a dummy interface to prevent errors.

Ideally this should go away as soon as unmanaged hosts are a different
class or Host::Base, moving validations and other particularities
to Host::Managed.