Add PDF documentation build

This commit adds PDF documentation build target 'pdf-docs' that will
build PDF versions of our docs. As per the Train community goal:

 https://governance.openstack.org/tc/goals/selected/train/pdf-doc-generation.html

Change-Id: Iecb0fe5b957af7dae66bea04dfbd9c2fb4f74a99
Story: #2006070
Task: #35456
diff --git a/doc/source/conf.py b/doc/source/conf.py
index 9059f8c..56043ba 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -165,7 +165,7 @@
 # Grouping the document tree into LaTeX files. List of tuples
 # (source start file, target name, title, author, documentclass [howto/manual]).
 latex_documents = [
-  ('index', 'DevStack-doc.tex', u'DevStack Docs',
+  ('index', 'doc-devstack.tex', u'DevStack Docs',
    u'OpenStack DevStack Team', 'manual'),
 ]