I have also tried ssh-ing, under root, into the fully qualified domain name — and it works.
I get this when I try in Ambari with the FQDN:
==========================
Creating target directory…
==========================
Command start time 2015-01-14 16:06:38
Warning: Permanently added ‘d00278.parknet-ad.pmh.org’ (ECDSA) to the list of known hosts.
Permission denied (publickey,password).
SSH command execution finished
host=d00278.parknet-ad.pmh.org, exitcode=255
Command end time 2015-01-14 16:06:38
ERROR: Bootstrap of host d00278.parknet-ad.pmh.org fails because previous action finished with non-zero exit code (255)
ERROR MESSAGE: Warning: Permanently added ‘d00278.parknet-ad.pmh.org’ (ECDSA) to the list of known hosts.
Permission denied (publickey,password).
STDOUT:
Warning: Permanently added ‘d00278.parknet-ad.pmh.org’ (ECDSA) to the list of known hosts.
Permission denied (publickey,password).