RST directive to discover and generate drivers doc

This patchset introduces a new custom directive called 'drivers-doc'
which loads all available drivers under a given namespace and import
their respective docstring into the .rst document.

This patchset also contains some modification/addition to the
docstring of these drivers to make the final document complete.

Change-Id: Ib3df59fa45cea9d11d20fb73a5f0f1d564135bca
Closes-Bug: #1536218
Closes-Bug: #1536735

watcher/decision_engine/planner/default.py

@@ -28,6 +28,13 @@ LOG = log.getLogger(__name__)
 
 
 class DefaultPlanner(base.BasePlanner):
+    """Default planner implementation
+
+    This implementation comes with basic rules with a fixed set of action types
+    that are weighted. An action having a lower weight will be scheduled before
+    the other ones.
+    """
+
     priorities = {
         'nop': 0,
         'sleep': 1,