Changed Stackforge explanation for retirement

Since Stackforge is retiring, two slides: 'What is
Stackforge for' and 'How to add a project to Stackforge'
is not needed for training materials.

Stackforge is currently in migrating status, and
it is used before TC's decision on August 2015,
IMO it is better to keep 'Stackforge' slide.
So some content related to Stackforge was modified
with Stackforge retired and current status.

Change-Id: Ia7026acfde12d129781d90e575bb3caa3d6609aa
Closes-Bug: #1490471

doc/upstream-training/source/04-program-ecosystem.rst

@@ -87,25 +87,9 @@ Stackforge
 .. image:: ./_assets/04-05-stackforge.png
   :width: 100%
 
-- Gerrit code review
-- Jenkins continuous integration
-- Git repository
-- IRC bots, pypi uploads...
-
-What is Stackforge for
-======================
-
-.. image:: ./_assets/04-06-stackforge.png
-  :width: 100%
-
-- Maintain related unofficial projects
-- Same tools and procedures used by the main projects
-
-How to add a project to Stackforge
-==================================
-
-.. image:: ./_assets/04-07-stackforge.png
-  :width: 100%
+- Stackforge was used for new unofficial projects, but retired on Aug 2015
+- Replaced by Big Tent initiative
+- Used the same processes and tools as the main OpenStack projects
 
 Exercise
 ========