Remove references to Big Tent from project list slides

This commit removes the references to the Big Tent from the project list
slides. We're trying to move away from that term because it causes
confusion to a lot of people. Instead we should call them OpenStack
projects. One note about big tent is left in, because people still use
the term, but we should use that to explain it's the old way to describe
all OpenStack projects.

Change-Id: Ifea66e1dae9c4e644e3efa70c0be0526b24d41cc

doc/upstream-training/source/slides/howitsmade-official-projects.rst

@@ -21,10 +21,10 @@ Official OpenStack projects
  - Testable on its own
  - Dashboard (horizon) and DevStack provide plugin interface for easy
    integration
- - Core and mature Big Tent projects are tracked in the
+ - Official OpenStack projects are tracked in the
    `Project Navigator <https://www.openstack.org/software/project-navigator>`_
 
-- Big Tent is a code name for the official OpenStack projects
+- Big Tent was a code name for the official OpenStack projects
 
  .. note::
 
@@ -67,7 +67,7 @@ Core and Optional Services
 Core and Optional Services
 ==========================
 
-.. image:: ./_assets/big-tent-and-core-services.png
+.. image:: ./_assets/official-openstack-projects.png
   :scale: 90 %