[Review] Request from 'dmajda' @ 'SUSE/machinery/global-json-schema-2'

Please review the following changes:
  * d93adc7 Implement JSON Schema validation of system descriptions
  * c3de350 Don't save version metadata for descriptions without format version
  * 14c95d1 Extract SystemDescription#validate_json
  * 4746b33 Use "class << self" syntax for methods in SystemDescription