Add a dynamic loading of the Watcher Planner implementation

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, ...).

The objective of this patchset is to give the ability to extend the
default set of planner algorithms currently available in Watcher
using Stevedore.

The doc need to explain how create a new planner.

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

Change-Id: I2fd73f8c4a457ee391d764a7a3f494deecd2634f

watcher/decision_engine/planner/default.py

@@ -16,9 +16,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
-from oslo_log import log
 
-from enum import Enum
+import enum
+from oslo_log import log
 
 from watcher._i18n import _LW
 from watcher.common import exception
@@ -37,7 +37,7 @@ LOG = log.getLogger(__name__)
 # https://wiki.openstack.org/wiki/NovaOrchestration/WorkflowEngines​
 
 
-class Primitives(Enum):
+class Primitives(enum.Enum):
     LIVE_MIGRATE = 'MIGRATE'
     COLD_MIGRATE = 'MIGRATE'
     POWER_STATE = 'POWERSTATE'

watcher/decision_engine/planner/manager.py

@@ -0,0 +1,53 @@
+# -*- encoding: utf-8 -*-
+# Copyright (c) 2016 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 oslo_config import cfg
+from oslo_log import log
+
+from watcher.decision_engine.planner.loading import default as loader
+
+
+LOG = log.getLogger(__name__)
+CONF = cfg.CONF
+
+default_planner = 'default'
+
+WATCHER_PLANNER_OPTS = {
+    cfg.StrOpt('planner',
+               default=default_planner,
+               required=True,
+               help='The selected planner used to schedule the actions')
+}
+planner_opt_group = cfg.OptGroup(name='watcher_planner',
+                                 title='Defines the parameters of '
+                                       'the planner')
+CONF.register_group(planner_opt_group)
+CONF.register_opts(WATCHER_PLANNER_OPTS, planner_opt_group)
+
+
+class PlannerManager(object):
+    def __init__(self):
+        self._loader = loader.DefaultPlannerLoader()
+
+    @property
+    def loader(self):
+        return self._loader
+
+    def load(self):
+        selected_planner = CONF.watcher_planner.planner
+        LOG.debug("Loading {0}".format(selected_planner))
+        return self.loader.load(name=selected_planner)