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!
@demobox @zack-shoylev Yesterday, I prototyped changing all collection related `size()` methods from/to:
- `size() > 0` -> `!isEmpty()`
- `size() == 0` ->  `isEmpty()`
- `size() != 0` -> `!isEmpty()`
- `Iterables.size(collection) > 0` -> `Iterables.isEmpty(collection)`

WDYT? If using `isEmpty()` is preferred, then we should document this in the best practices!