Document how/where to get dependencies to DIB

[Turbo Fredriksson]

URL: http://docs.openstack.org/developer/trove/dev/building_guest_images.html

* It only states to clone "diskimage-builder", but in the section below, it's using parts/elements from "tripleo-image-elements" (amongst others?).
  => Please provide 'git clone' command examples to retrieve all dependencies used later in the example command(s).
  NOTE: It does say a page or two below that "In addition, projects like TripleO [5] provide elements as well" (with links to these projects).
        Keyword here is "in addition" and "as well", indicating that it's not _required_ (which it seems to be, when one dissects the "disk-image-create" command line example).

* In the example command, it references "$REDSTACK_SCRIPTS", "$PATH_TRIPLEO_ELEMENTS" and "$PATH_DISKIMAGEBUILDER} (etc, etc), but they are not set previously.
  => Please show how to install the different parts and then set the appropriate environment variables (or even better, just set the "ELEMENTS_PATH" variable with these paths where they where installed).

* The command line example

    local QEMU_IMG_OPTIONS=$(! $(qemu-img | grep -q 'version 1') && \
        echo "--qemu-img-options compat=0.10")

  is missing a '--version' in the "qemu-img" part.

* In the "final" "disk-image-create" command line example, it's using (unset) variables as parameters. What are these options?
  => Please refrain from using variables as much as possible.