Moved global constants to class constants
Missed a couple SYSTEM_ prefixes
Added SSH_AGENT_FAILURE constant back in
Merge pull request #524 from cnelissen/SystemPackageConstantsFix

System package constants fix

* cnelissen/SystemPackageConstantsFix:
  Added SSH_AGENT_FAILURE constant back in
  Missed a couple SYSTEM_ prefixes
  Moved global constants to class constants