Switch to "tox -e docs"

Use the new PTI docs building commands, these use "tox -e docs" for
building, also update the publishing job.

Update tox.ini to use sphinx-build directly as documented in the PTI.

Note that we have to disable warnings, otherwise building fails with:
doc/source/results/ocata/ptl.rst:3:Duplicate explicit target name: "tony breeds (tonyb)".
Be aware that with current pbr and Sphinx, warnings are disabled due to
change of parameter names - so, this keeps the status quo.

Depends-On: https://review.openstack.org/615501
Needed-By: https://review.openstack.org/615637
Change-Id: I4713f3a25e08056b7bce9812fa2cf62119330fcc

.zuul.d/project.yaml

@@ -1,6 +1,6 @@
 - project:
     templates:
-      - build-openstack-sphinx-docs
+      - build-openstack-docs-pti
       - openstack-python35-jobs
       - openstack-python36-jobs
     check:
@@ -15,4 +15,4 @@
         - election-tox-ci-checks
     post:
       jobs:
-        - publish-static
+        - publish-tox-docs-static

tox.ini

@@ -27,7 +27,9 @@ basepython = python2
 commands = {posargs}
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+# NOTE: -W is not added since ocata/ptl.rst throws a warning about duplicate
+# IDs.
+commands = sphinx-build -b html -d doc/build/doctrees doc/source doc/build/html
 
 [testenv:ci-checks]
 commands = ci-check-all-candidate-files