Add a common generic dynamic loader for watcher

In watcher, an audit generates a set of actions which
aims at achieving a given goal (lower energy consumption, ...).
It is possible to configure different strategies in order to achieve
each goal. Each strategy is written as a Python class which produces
a set of actions. Today, the set of possible actions is fixed for a
given version of Watcher and enables optimization algorithms to
include actions such as instance migration, changing hypervisor state,
changing power state (ACPI level, ...).

This patchset add a common generic dynamic loader for plugins,
such as for custom Actions, Strategies, Planners, etc.

Partially implements: blueprint watcher-add-actions-via-conf

Change-Id: I59d031b93865fff2540e3973921e1bdafa95f88e

watcher/tests/common/loader/FakeLoadable.py

@@ -0,0 +1,28 @@
+# -*- encoding: utf-8 -*-
+# Copyright (c) 2015 b<>com
+#
+# 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 FakeLoadable(object):
+    @classmethod
+    def namespace(cls):
+        return "TESTING"
+
+    @classmethod
+    def get_name(cls):
+        return 'fake'

watcher/tests/common/loader/test_loader.py

@@ -0,0 +1,53 @@
+# -*- encoding: utf-8 -*-
+# Copyright (c) 2015 b<>com
+#
+# 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 __future__ import unicode_literals
+
+import mock
+
+from oslotest.base import BaseTestCase
+from stevedore.driver import DriverManager
+from stevedore.extension import Extension
+
+from watcher.common import exception
+from watcher.common.loader.default import DefaultLoader
+from watcher.tests.common.loader.FakeLoadable import FakeLoadable
+
+
+class TestLoader(BaseTestCase):
+    @mock.patch("watcher.common.loader.default.DriverManager")
+    def test_load_driver_no_opt(self, m_driver_manager):
+        m_driver_manager.return_value = DriverManager.make_test_instance(
+            extension=Extension(name=FakeLoadable.get_name(),
+                                entry_point="%s:%s" % (
+                                FakeLoadable.__module__,
+                                FakeLoadable.__name__),
+                                plugin=FakeLoadable,
+                                obj=None),
+            namespace=FakeLoadable.namespace())
+
+        loader_manager = DefaultLoader(namespace='TESTING')
+        loaded_driver = loader_manager.load(name='fake')
+
+        self.assertEqual(loaded_driver.get_name(), FakeLoadable.get_name())
+
+    @mock.patch("watcher.common.loader.default.DriverManager")
+    def test_load_driver_bad_plugin(self, m_driver_manager):
+        m_driver_manager.side_effect = Exception()
+
+        loader_manager = DefaultLoader(namespace='TESTING')
+        self.assertRaises(exception.LoadingError, loader_manager.load,
+                          name='bad_driver')