Fixed Bug:1400991 - Modified .off() logic for change events for dropdown.
ISSUE: The dropdowns had stopped working at Server-Manager due to previous fix where .off() handlers were introduced to stop triggering of multiple select2 change events
FIX: .off events are applied selectively and it just disables the change function and rest of the events remain unaffected

Change-Id: Iec7daf03810dfdeff0dfdb8b44eee99f488c74e8