add a way for callback to disable itself

The idea is that some plugin would not be called in some
specific case, and the callback should decide by itself.

Having a way to globally disable it is much cleaner than
disabling every method one by one on the plugin side.

My use case is for fedora-infrastructure that cannot be run
from git checkout since it try to connect to the message bus,
but another case would be to bootstrap infrastructure, or to
run the code on a test servers without having all the callback
infrastructure setup.

lib/ansible/callback_plugins/noop.py

@@ -21,11 +21,16 @@ class CallbackModule(object):
     """
     this is an example ansible callback file that does nothing.  You can drop
     other classes in the same directory to define your own handlers.  Methods
-    you do not use can be omitted.
+    you do not use can be omitted. If self.disabled is set to True, the plugin
+    methods will not be called.
 
     example uses include: logging, emailing, storing info, etc
     """
 
+    def __init__(self):
+        #if foo:
+        #    self.disabled = True
+        pass
 
     def on_any(self, *args, **kwargs):
         pass