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/actions/sleep.py

@@ -28,6 +28,16 @@ LOG = log.getLogger(__name__)
 
 
 class Sleep(base.BaseAction):
+    """Makes the executor of the action plan wait for a given duration
+
+    The action schema is::
+
+        schema = Schema({
+         'duration': float,
+        })
+
+    The `duration` is expressed in seconds.
+    """
 
     DURATION = 'duration'