Join EnterpriseChef and Open Source Chef

In Chef 12 all flavors have been merged and open sourced. Starting from version 12, the open source Chef Server comes with all features from Enterprise Chef, including the Organization, User and Permissions APIs.

Now that it is all part of the same API, the EnterpriseChef provider should be removed and its specific APIs added to the ChefApi. To be backward compatible, it could be added as a @Delegate api that returns an Optional object which contents would be populated depending on the api-version field.