At https://github.com/storpool/addon-sshagent is my interpretation how to setup a ssh-agent service for OpenNebula on the front end, following the final note at the end of Step 4(Configure Passwordless SSH) of the KVM Node Installation guide.
The idea is to define a ssh-agent service that will be started automatically when the opennebula service is started and provide a ssh-agent socket at a predefined hard-coded path. Exporting the SSH_AUTH_SOCK variable to the environment will guarantee that the ssh sessions initiated by the OpenNebula modules will use the socket to use and forward the ssh-agent loaded credentials to the hosts.
In ~oneadmin/.ssh/config I am dropping a line to tell the ssh client to load the ssh keys to the agent. This should work for all default keys recognizable by the ssh.