[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_140811_add_data_format_attribute'

Please review the following changes:
  * a603de1 Describe format version policy in the data model description
  * 6255347 Marks descriptions with incompatible data format in "machinery list"
  * 1c61403 Raise error if the user tries to load a description with old data format
  * f037235 Add a format_version attribute to the description meta data
  * 74e501f Improve coding style and readability