Prepare for dynamic generation of tempest plugin registry

This is based on the devstack plugin detection code, modified
to check setup.cfg for a tempest_plugins string.  It is to be
executed from a periodic proposal job, generating a list of
plugins in the same format.

Change-Id: I53d082319b6af6fb47083ea42c3ff6b109882e72
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 17def1c..f5de309 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -11,6 +11,7 @@
    HACKING
    REVIEWING
    plugin
+   plugin-registry
    library
 
 ------------