Add an overview diagram to the userguide

The diagram covers the tools used to configure the base image for use as
a TripleO instance, both pre and post build time. It mentions
diskimage-builder, tripleo-image-elements, os-collect-config,
os-refresh-config, os-apply-config and Heat.

Change-Id: I68b8f90efd0f92a8a35baa53733bbc2a7145cefc
Merge "Add an overview diagram to the userguide"