agent wait-for install-complete implementation

A cli subcommand that waits for the cluster to come up. This should be able to reuse the code from the regular openshift-install wait-for install-complete command largely unchanged, but if the k8s API is not available it may be because we're still running the assisted part of installation. It probably needs to fall back to checking for that. I'm not sure what assumptions in the existing installer command about when it is safe to run it. Ideally we would keep behaviour relatively consistent.