pre-populate /srv/tftpboot repos in overlay filesystem

The /srv/tftpboot/repos in the overlay are now pre-populated by
bind-mounting from the image-building host.
add build-custom to run at end of appliance creation
the RAM disk needs at least 5GB

... and that's without the software repositories embedded.
mention packer
fill out image-building instructions (WIP)
tmpfs RAM disk needs a LOT when all the repos are overlaid
allow passing parameters to kiwi (e.g. --type)
switch to .vmdk for virtualbox
fix to save .vmdk files
initial stab at a Vagrantfile
import qcow2box.sh

copied from
https://github.com/SUSE/cloud/blob/master/dev-setup/wip-vagrant/scripts/generate-box.sh

In general I *hate* copying code around, but in this case the repository
needs to be self-contained.  Hopefully Thomas or I can turn this script
into a proper independent github project at some point.
don't hardcode Vagrantfile and metadata.json within script
fix ANSI colour sequences
improve variable naming and fix relative paths for input
clarify that actions are in progress
noone uses 2-column indent for shell ;-)

not even most crazy Rubyists ;-)
ensure run as root
use first partition of .qcow2
don't bother unloading nbd - something else might be using it too
wrap code in functions for clarity