JCLOUDS-245: Refactored model to immutable

Refactored the domain model to be immutable and addressed some
inconsistences with the Chef Server API model.

Also removed all HEAD methods (objectExist) as they are no longer
supported in Enterprise Chef or Chef 11. They are replaced by the
corresponding GET calls to fetch the entire objects from the api.

Now all live tests are passing for Community Chef 0.10.8, 11.0.6 and
Enterprise Chef.