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!
I will try to do some database-specific refactoring and cleanup very soon. Hopefully these can be addressed in separate PRs?