Add an authentication plugin framework

This change adds an authentication plugin framework to be used by
the PyKMIP server. This framework will allow the server to query
third-party authentication systems for user identity information,
improving the access control model for the server. The initial
plugin provided queries an instance of the new SLUGS library.

kmip/services/server/auth/api.py

@@ -0,0 +1,45 @@
+# Copyright (c) 2018 The Johns Hopkins University/Applied Physics Laboratory
+# All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+import abc
+import six
+
+
+@six.add_metaclass(abc.ABCMeta)
+class AuthAPI:
+    """
+    The base class for an authentication API connector.
+    """
+
+    @abc.abstractmethod
+    def authenticate(self,
+                     connection_certificate=None,
+                     connection_info=None,
+                     request_credentials=None):
+        """
+        Query the configured authentication service with the given credentials.
+
+        Args:
+            connection_certificate (cryptography.x509.Certificate): An X.509
+                certificate object obtained from the connection being
+                authenticated. Optional, defaults to None.
+            connection_info (tuple): A tuple of information pertaining to the
+                connection being authenticated, including the source IP address
+                and a timestamp (e.g., ('127.0.0.1', 1519759267.467451)).
+                Optional, defaults to None.
+            request_credentials (list): A list of KMIP Credential structures
+                containing credential information to use for authentication.
+                Optional, defaults to None.
+        """