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).

The related [PR 463](https://github.com/jclouds/jclouds/pull/463) in the jclouds/jclouds project will need to be merged first!

I think we agreed on removing nullable here?