The Demo Environment slides have an exercise to make sure that
DevStack is checked out to the VM where they wish to run DevStack
but have no instructions for doing so. I added a slide with the
steps for cloning, configuring and running devstack.
I also improved the wording of the exercise on the final slide.
The details on the slide for using 'screen' were pretty light so
I also added content there.
Change-Id: I96a623323020a630d88d5f6d8659a6d49bd44ec7
5a0abcc467