Fix duplicate callback issue in v2

All v2+ callbacks can now optionally define a CALLBACK_TYPE, which
when set to 'stdout' will limit those callbacks which are used for
primary output to a single callback plugin (specified to the
TaskQueueManager object and configurable in ansible.cfg/environment)

v2/ansible/plugins/callback/default.py

@@ -31,6 +31,7 @@ class CallbackModule(CallbackBase):
     '''
 
     CALLBACK_VERSION = 2.0
+    CALLBACK_TYPE = 'stdout'
 
     def v2_on_any(self, *args, **kwargs):
         pass