Service discovery failed for this host: Agent exited with code 255: Pseudo-terminal will not be allocated because stdin is not a terminal. Host key verification failed.
Current Setup: Ubuntu 14.04 running OMD
Solution: Running the ssh command from the OMD user appears to fix the problem by allowing them into the known hosts list.
then run your remote command eg:
ubuntu_vm[[omd-site-name]]:~$ ssh -l root -i /etc/check_mk/check_mk.key [ip]
It will ask to save the host to the known hosts, type yes and enter. It should then output the status from the check_mk agent.