Use the oslo.config sphinx module to generate sample

This commit switches to using the oslo.config sphinx extension to
generate a sample config instead of doing it manually in the docs'
conf.py module.

Change-Id: I7032cb4f56c728911ffa5c6911eb504f53bc80af
diff --git a/.gitignore b/.gitignore
index f584532..efba45e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -19,4 +19,4 @@
 .coverage*
 !.coveragerc
 cover/
-doc/source/_static/tempest.conf
+doc/source/_static/tempest.conf.sample