Change chown back to rootfs.tar and md5 file. No need for root.
Change naming scheme for images, update docs.

Images are now as follows:
busybox -> base
busyboxplus -> git + cURL
busyboxplus-curl -> cURL only

Reasoning: git included all of curl anyway, so with that and the fact
that it's simpler to use a git repository to store configuration for a
cluster (via git) instead of try to use cURL (which would need
etcd/confd, and/or a bunch of other tools to make work) I decided make
git the default hub-base image and use git as the main method of
extracting configuration.