Prefer Regions to Zones in Neutron v2

This is an addendum to PR #111 which introduced the `getConfiguredRegions()` method in the Neutron v2 API.  This PR updates all references from `Zone` to `Region` in an effort to move towards common OpenStack nomenclature.

- Change API methods from `get*ApiInZone(...)` to `get*ApiInRegion(...)`
- Replace `@Zone` with `@Region`
- Replace `ZoneToEndpoint` with `RegionToEndpoint`
- Update all live/mock tests
I guess the intention will be to name the methods `get...ExtensionApi` for **all** extension APIs at some point (e.g. also in openstack-nova)..?