Prefer OpenStack Regions

This PR removes all references to "zone" in the OpenStack APIs in favor of the common "region" nomenclature as described in [JCLOUDS-647](https://issues.apache.org/jira/browse/JCLOUDS-647).

This PR will need a squash after its reviewed!
> If you didn't see any issues (can't really imagine why you would see any), I say yes to all of that.

+1, unsurprisingly ;-) I think it should indeed be recommended because "isEmpty" can be a simple check (basically !hasNext), but calculating the size can be much more expensive.