When instantiating VM's through OpenNebula, when logging into those VM's, the VM's may ask you for a password. The solution here is to edit the template in the GUI and add a context to it including the public SSH key. Click on the template then Update from the top right hand side buttom row then enter the key in:
If the above field is empty, ensure it has the proper passless SSH key. Further reading on contextualization.
The other reason for this issue is that you are trying to get into your machine as opennebula. Try as root instead:
[oneadmin@opennebula01 .ssh]$ ssh 192.168.0.100
[oneadmin@opennebula01 .ssh]$ ssh firstname.lastname@example.org