[JENKINS-16957] Tracker field validations not working in CollabNet plugin


* src/main/java/hudson/plugins/collabnet/tracker/CNTracker.java
  (doCheckTracker): Use updated argument list style
  (doCheckAssign): Use updated argument list style and change to doCheckAssignUser to match the
  field name.

* src/main/java/hudson/plugins/collabnet/util/CNFormFieldValidator.java
  (trackerCheck, assignCheck): reflect changes above. And for assignCheck fix the project
  lookup to be by the project name/title.