add rake to kitchen  Gemfile
add rake cloud:vm:deregister task
deregister VM before terminating
deregister VM via rake task
fix credentials constant
call read on Credentials class
clean up Rakefile
Merge pull request #48 from SUSE/review_140410_add_deregister_rake_task_and_fix_suseconnect_jenkins_node

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