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/applier/workflow_engine/default.py

@@ -28,6 +28,12 @@ LOG = log.getLogger(__name__)
 
 
 class DefaultWorkFlowEngine(base.BaseWorkFlowEngine):
+    """Taskflow as a workflow engine for Watcher
+
+    Full documentation on taskflow at
+    http://docs.openstack.org/developer/taskflow/
+    """
+
     def decider(self, history):
         # FIXME(jed) not possible with the current Watcher Planner
         #