PDF Documentation Build tox target

This patch adds a `pdf-docs` tox target 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

Add sphinxcontrib-svg2pdfconverter to doc/requirements.txt
to convert our SVGs.

Change-Id: I6f3b5967ad906632d325b002b510e3fa96aac227
Story: 2006105

tox.ini

@@ -35,12 +35,22 @@ setenv =
 [testenv:docs]
 basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
-commands=
+commands =
     bash -c "rm -rf doc/build"
     doc8 doc
     sphinx-build -b html doc/source doc/build/html
 
 
+[testenv:pdf-docs]
+basepython = python3
+deps = {[testenv:docs]deps}
+whitelist_externals =
+  make
+commands =
+  sphinx-build -W -b latex doc/source doc/build/pdf
+  make -C doc/build/pdf
+
+
 [doc8]
 # Settings for doc8:
 extensions = .rst