change bootstrap to pivot

See this thread, specifically this comment.

TL;DR - today the installer launches a bootimage, which is usually the pinned RHCOS version (for IPI installs), but can be different - see this issue which is about making it easier for people to find the correct bootimage.

See also https://github.com/openshift/installer/pull/2532

Filing this issue to track changing the installer to do a pivot on the bootstrap node.

I think the architecture would look like a new bootstrap-pivot.service between release-image.service and bootkube.service - we'd run the MCD on the bootstrap host, telling it to pivot to the target machine-os-content.

In fact...if we did this, we could streamline down the bootimages - e.g. no reason to ship kubelet/cri-o in the bootimages. (Though, we'd need new terminology like "bootRHCOS" to distinguish from "normal" RHCOS in machine-os-content or so?) Anyways, not required for this change.

Start with the linked enhancement discussion and installer PR #2532, then trace how release-image.service and bootkube.service are coordinated. Define what a bootstrap-pivot.service using the MCD to pivot to the target machine-os-content must do, and consider the stated bootimage terminology implications; the issue is done when the installer performs that bootstrap pivot reliably.