Build pdf doc

The is one of community goals that each project could produce a
single PDF file. The pdf should be in the output of openstack-tox-docs
job.

TeX packages are required to build PDF locally, following is recommended:

* inkscape
* texlive-latex-base
* texlive-latex-extra
* texlive-fonts-recommended

More about the goal:
https://governance.openstack.org/tc/goals/train/pdf-doc-generation.html
https://etherpad.openstack.org/p/train-pdf-support-goal
https://etherpad.openstack.org/p/pdf-goal-train-common-problems

Change-Id: I8ebbe9ea1bb2914061a8307f3edfabab166a3c38
diff --git a/tox.ini b/tox.ini
index d686ee9..c81582c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -28,6 +28,12 @@
 basepython = python3
 commands = python setup.py build_sphinx
 
+[testenv:pdf-docs]
+basepython = python3
+whitelist_externals = make
+commands = sphinx-build -b latex doc/source doc/build/pdf
+           make -C doc/build/pdf
+
 [testenv:debug]
 basepython = python3
 commands = oslo_debug_helper {posargs}