OpenShift CoreOS Layering (provisional)

**NOTE: Nothing in this proposal should be viewed as final.
It is highly likely that details will change.  It is quite possible
that larger architectural changes will be made as well.**

Change RHEL CoreOS as shipped in OpenShift to be a "base image" that can
be used as in layered container builds and then booted.  This will allow
custom 3rd party agents delivered via RPMs installed in a container
build.  The MCO will roll out and monitor these custom builds the same
way it does for the "pristine" CoreOS image today.

This is the OpenShift integration of [ostree native containers](https://fedoraproject.org/wiki/Changes/OstreeNativeContainer) or [CoreOS layering](coreos/enhancements#7) via the MCO.