Add cluster setup documentation for OpenStack.

The EC2 documentation makes it very easy for new users to spin up CoreOS
clusters, but the OpenStack docs focused on single instances. I modified the
OpenStack docs to help walk new users through creating a cluster. Portions were
copied (adding more machines & multiple clusters), and slightly modified to use
OpenStack terminology.

I also split long `nova` & `glance` commands across multiple lines to help with
readability and modification.