Raise NotImplementedError instead of NotImplemented

NotImplementedError is the name of the exception
(https://docs.python.org/2/library/exceptions.html).
NotImplemented is the name of a constant
(https://docs.python.org/2/library/constants.html).
This patch fix it.

Change-Id: If0c220b8fc8480e5904400e9086935944bce728a
Closes-Bug: #1339855

keystonemiddleware/auth_token/__init__.py

@@ -407,7 +407,7 @@ class BaseAuthProtocol(object):
         :returns: The token data
         :rtype: dict
         """
-        raise NotImplemented()
+        raise NotImplementedError()
 
     def process_response(self, response):
         """Do whatever you'd like to the response.