Add a new installer subcommand, openshift-install agent create config-image.

The should create a small ISO (i.e. not a CoreOS boot image) containing just the configuration files from the automation flow:

• rendezvousIP config file
• ClusterDeployment manifest
• AgentPullSecret manifest
• AgentClusterInstall manifest
• TLS certs for admin kubeconfig
• password hash for kubeadmin console password
• NMStateConfig
• extra manifests
• hostnames
• hostconfig (roles, root device hints)
• ClusterImageSet manifest (for version verification)

The contents in the disk could be in any format, but should be optimised to make it simple for the service in AGENT-562 to read.