Update the server session to use the auth plugin framework

This change updates how the server session handles message
processing, adding support for the new authentication plugin
framework. Session unit tests have been updated to account for
this change.

kmip/services/server/engine.py

@@ -335,9 +335,6 @@ class KmipEngine(object):
         else:
             auth_credentials = None
 
-        # TODO (peter-hamilton) This is a shim until SLUGS integration is done.
-        credential = [credential, None]
-
         self._verify_credential(auth_credentials, credential)
 
         # Process the batch error continuation option