Ambari 2.5.1.0 / centOS-7 -> Ambari agent, Ambari server running with an external postgres9.6 database on same system.
Now, hit Ambari url ->
login to Ambari -> Create a cluster -> provide cluster name -> next
selecet HDP-2.6.3.0 -> select 'use public repository' -> select redhat7 OS -> next
Enter only one hostname / FQDN of current centOS-7 machine.
Select 'Perform manual registration on hosts and do not use SSH'
Now Click 'Register and Confirm' button
Result: "status - failed" with message "Host checks were skipped on 1 hosts that failed to register."