Add (unused) arg to _OfflineClusterMerge

cli._RunWhileClusterStoppedHelper.Call passes (self, *args) to functions
called via cli.RunWhileClusterStoppedHelper(). The code in cluster-merge
was broken by commit d8aab233.

Signed-off-by: Stephen Shirley <diamond@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>