I'm not sure whether this is a bug, but I think so.
I just Kerberized HDP 3.1, using Ambari 2.7.3. In 2.6.3 I didn't need to register the Ambari host as a member of the cluster. In 2.7.3 I do. If the ambari host isn't registered the host check fails claiming "host not found.' Initially I thought there was a DNS or LDAP issue, but in fact it failed to find the hostname in Ambari's internal database. The only way I could find to fix it was to register the host, which meant running at least the metrics daemon on it.
I don't believe the Ambari host should need to be a cluster member. If it is necessary, a more explicit error messge would be useful, since "host not found" sounds more like a problem with /etc/hosts or DNS.