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

Story: 2006122
Task: 35513
Change-Id: I89db230c31464a89a82aa72b86348f51b61990ca

.zuul.yaml

@@ -8,9 +8,6 @@
         - test-release-openstack: &release_overrides
             vars:
               build_wheel: false
-        - openstack-tox-docs: &doc_overrides
-            vars:
-              tox_skip_pdf: true
         - openstack-tox-py35: &py35_overrides
             vars:
               tox_envlist: py35-compilelibs
@@ -29,7 +26,6 @@
     gate:
       jobs:
         - test-release-openstack: *release_overrides
-        - openstack-tox-docs: *doc_overrides
         - openstack-tox-py35: *py35_overrides
         - openstack-tox-py36
         - openstack-tox-py37

tox.ini

@@ -34,6 +34,14 @@ deps =
 commands =
   sphinx-build -W -b html -d doc/build/doctrees doc/source doc/build/html
 
+[testenv:pdf-docs]
+deps = {[testenv:docs]deps}
+allowlist_externals =
+  make
+commands =
+  sphinx-build -W -b latex doc/source doc/build/pdf
+  make -C doc/build/pdf
+
 [flake8]
 # H101: Use TODO(NAME)
 # H404: multi line docstring should start without a leading new line