Bug 6995 - Change event notification subscription usability PART1

  * added leaf of notification-output-type to grouping for use
    both augmentations of notifications:
      * data-change notifications
      * yang notifications
  * implemented support of output type for data-change
    notifications (in draft02 and in latest draft)
  * fixed tests to support output of notification

Change-Id: I0b8b9803d1bd2daa7f319e9332ecaf1e96da16ab
Signed-off-by: Jakub Toth <jatoth@cisco.com>