[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_141015_fix_models'

Please review the following changes:
  * 47b5897 Fix models so that they make use of the specific model classes
  * 15434ff Extend the description factory to support the remaining scopes
  * 53fd0f6 Always treat the property classes as symbols
Is this fixing a bug? If yes, we should have a test which exposes the bug.