Update getting-started-compute.md

Stumbled across this while trying to get DevStack working with pkgcloud .... maybe it helps... people seldom complain about slight issues in the document... 
Made the few amendments (missing region argument, changed flavor and image name) I needed to do to get DevStack to run... not entirely sure if it also applies to full OpenStack (lack of iron on my part ;-) )
Maybe add a comment that this is devstack default