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!

@zack-shoylev The pattern of passing the `@EndpointParam` parser is the same throughout the OpenStack APIs. Passing null ends up throwing an `IllegalArgumentException` in the `*ToEndpoint` related functions in the [location package](https://github.com/jclouds/jclouds/tree/master/core/src/main/java/org/jclouds/location/functions).