Most cloud images support public key authentication rather than conventional username/password authentication. Before launching an instance, you must generate a public/private key pair using ssh-keygen and add the public key to your OpenStack environment.
If your host does not contain the public/private key pair created in an earlier step, SSH prompts for the password associated with the user.
Please refer-
http://docs.openstack.org/icehouse/install-guide/install/apt-debian/content/launch-instance-neutron.html (http://docs.openstack.org/icehouse/in...)