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

Please review the following changes:
  * c08c2a7 Extract JSON handling into a Manifest class
  * 2d52611 Get rid of SystemDescription#from_json method
  * d8a0df4 Split #from_json into three methods (parse, validate, objectify)