Support for CoreOS images with Docker Containers

diskimage-builder should support building CoreOS PXE images with embedded docker containers. Currently, when you build a ramdisk with diskimage-builder, there is no init running, and generally no way to treat it as a fully functional OS.

For the use case of the ironic-python-agent, we've found that embedding it in a Docker container, and running that container inside a CoreOS PXE image gives us the greatest flexibility.

I'd suggest it work this way:

CoreOS PXE ramdisk images would be a separate supported path than Fedora/Debian is currently. The 'elements' provided for a CoreOS PXE Ramdisk would be Docker containers on the public registry, which would be imported into the CoreOS image that is created.