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!
[minor] (and I know it was a simply rename PR) `!regions.isEmpty()`?