make v2_playbook_on_notify method signatures consistent

This was causing an exception in the TaskQueueManager when a third
party handler plugin was processed that inherited or explicitly
called the callback method from super because the method signature
was incorrect in callback/__init__ and it processed the arguments as
incorrect data types as a side effect.

Signed-off-by: Adam Miller <admiller@redhat.com>

lib/ansible/plugins/callback/__init__.py

@@ -348,8 +348,7 @@ class CallbackBase(AnsiblePlugin):
     def v2_playbook_on_start(self, playbook):
         self.playbook_on_start()
 
-    def v2_playbook_on_notify(self, result, handler):
-        host = result._host.get_name()
+    def v2_playbook_on_notify(self, handler, host):
         self.playbook_on_notify(host, handler)
 
     def v2_playbook_on_no_hosts_matched(self):