Add new convenience User method for authentication data change

Add new convenience User method for authentication data change

Also update createAndPromote.php to use it so it can display errors.

Note that there are two possible approaches. The other is to
do the update for all requests for which the test passed,
even if some tests fail. The approach of the patch seems
more manageable from the caller's point of view
(either the operation was a success, or it failed and
nothing happened).

Change-Id: I86abed4b80472cd888337444fac0cbcb870b1246