Add MicroversionMixin for microversion support

This generic mixin overloads plugin client() method to return
a client with max microversion unless a specific version is requested.

This provides two abstract methods
- get_max_microversion()
- is_version_supported()

for client plugins to override when supporting microversions.

Change-Id: I8873ab6d815671b6647b08578d1406dd874269f6

heat/engine/clients/microversion_mixin.py

@@ -0,0 +1,45 @@
+#
+#    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
+
+from heat.common import exception
+
+
+@six.add_metaclass(abc.ABCMeta)
+class MicroversionMixin(object):
+    """Mixin For microversion support."""
+
+    def client(self, version=None):
+        if version is None:
+            version = self.get_max_microversion()
+        elif not self.is_version_supported(version):
+            raise exception.InvalidServiceVersion(
+                version=version,
+                service=self._get_service_name())
+
+        if version in self._client_instances:
+            return self._client_instances[version]
+
+        self._client_instances[version] = self._create(version=version)
+        return self._client_instances[version]
+
+    @abc.abstractmethod
+    def get_max_microversion(self):
+        pass
+
+    @abc.abstractmethod
+    def is_version_supported(self, version):
+        pass