[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_141016_fix_models'

Please review the following changes:
  * f0d8b32 Extract Scope interface assertions into a shared_examples group
  * a855d9c Add method to the system description factory which return the raw json
  * b89118e Add test the shows that has_property works with string and symbols
  * 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