WIP: overlay node image before bootstrapping if necessary

[jlebon]

As per https://github.com/openshift/enhancements/pull/1637, we're trying to get rid of all OpenShift-versioned components from the bootimages.

This means that there will no longer be oc, kubelet, or crio binaries for example, which bootstrapping obviously relies on.

To adapt to this, the OpenShift installer now ships a new node-image-overlay.service in its bootstrap Ignition config. This service takes care of pulling down the node image and overlaying it, effectively updating the system to the node image version.

Here, we accordingly also adapt assisted-installer so that we run node-image-overlay.service before starting e.g. kubelet.service and bootkube.service.

See also: https://github.com/openshift/installer/pull/8742