[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
Yes, it is already implicitly tested in the model tests but I also think that having a dedicated one makes sense.