[Review] Request from 'vlewin' @ 'SUSE/connect/review_140410_add_deregister_rake_task_and_fix_suseconnect_jenkins_node'

Please review the following changes:
  * eb4b8bc call read on Credentials class
  * 288ae1c fix credentials constant
  * 7ae879e deregister VM via rake task
  * c15c3a8 deregister VM before terminating
  * 0d943c9 add rake cloud:vm:deregister task
  * c22f8b8 add rake to kitchen  Gemfile