Add metadata plugin mechanism

Until now we only supported storing resources in memory, and that
mechanism was tightly coupled with everything else.

This patch decouples the metadata storing into a plugin mechanism using
'Python entrypoints.  The default storing mechanism is still in memory,
and for now it's the only mechanism available.

The JSON serialization mechanism has not yet been adapted to the
metadata plugin mechanism, so it will only work with the default plugin.

cinderlib/exception.py

@@ -0,0 +1,29 @@
+# Copyright (c) 2018, Red Hat, Inc.
+# 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.
+
+from cinder import exception
+
+
+NotFound = exception.NotFound
+VolumeNotFound = exception.VolumeNotFound
+SnapshotNotFound = exception.SnapshotNotFound
+ConnectionNotFound = exception.VolumeAttachmentNotFound
+
+
+class InvalidPersistence(Exception):
+    __msg = 'Invalid persistence storage: %s'
+
+    def __init__(self, name):
+        super(InvalidPersistence, self).__init__(self.__msg % name)