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

doc/source/dev/plugins.rst

@@ -0,0 +1,38 @@
+..
+      Except where otherwise noted, this document is licensed under Creative
+      Commons Attribution 3.0 License.  You can view the license at:
+
+          https://creativecommons.org/licenses/by/3.0/
+
+
+=================
+Available Plugins
+=================
+
+.. _watcher_strategies:
+
+Strategies
+==========
+
+.. drivers-doc:: watcher_strategies
+
+.. _watcher_actions:
+
+Actions
+=======
+
+.. drivers-doc:: watcher_actions
+
+.. _watcher_workflow_engines:
+
+Workflow Engines
+================
+
+.. drivers-doc:: watcher_workflow_engines
+
+.. _watcher_planners:
+
+Planners
+========
+
+.. drivers-doc:: watcher_planners