Some of the openstack icons don't render properly

[lrjball]

Some of the openstack icons currently don't work as expected. This looks to be because the code to generate the _icon_dir assumes the directories under each provider contain all of the pngs, and don't themselves contain any directories. For some of the openstack icons the pngs are too nested so one of the directories gets missed off the _icon_dir, leading to the png not being found.

For example, diagrams.openstack.deployment.Chef does not currently work and neither do the other icons in that folder, as well as a few other folders. I think the easiest fix is to just remove the excessive nested, so will put in a PR for that shortly.