Updated DE architecture doc + 'period' param

In this changeset, I updated the architecture documentation
about the Decision Engine by adding a new sequence diagram
outlining the CDM synchronization workflow.
I also explained the default 'period' parameter used in the
CDMC plugin.

Change-Id: I09790281ba9117e302ab8e66a887667929c6c261
Partially-Implements: blueprint cluster-model-objects-wrapper

doc/source/dev/plugin/planner-plugin.rst

@@ -91,8 +91,10 @@ tune the planner to its needs. To do so, you can implement the
             assert self.config.test_opt == 0
             # [...]
 
-        def get_config_opts(self):
-            return [
+        @classmethod
+        def get_config_opts(cls):
+            return super(
+                DummyPlanner, cls).get_config_opts() + [
                 cfg.StrOpt('test_opt', help="Demo Option.", default=0),
                 # Some more options ...
             ]