Follow the new PTI for building docs

For compliance with the Project Testing Interface as described in:
https://governance.openstack.org/tc/reference/project-testing-interface.html#documentation
http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html
http://lists.openstack.org/pipermail/openstack-dev/2018-March/128594.html

Change-Id: I86d2460231d246005cfdace75c50dc54c137f0ca

tox.ini

@@ -43,13 +43,12 @@ commands =
 [testenv:docs]
 basepython = python3
 setenv = PYTHONHASHSEED=0
-commands =
-    doc8 doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
-    python setup.py build_sphinx
+deps = -r{toxinidir}/doc/requirements.txt
+commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:api-ref]
-# This environment is called from CI scripts to test and publish
-# the API Ref to developer.openstack.org.
+basepython = python3
+deps = -r{toxinidir}/doc/requirements.txt
 whitelist_externals = bash
 commands =
   bash -c 'rm -rf api-ref/build'
@@ -93,6 +92,7 @@ ignore-path=doc/source/image_src,doc/source/man,doc/source/api
 
 [testenv:releasenotes]
 basepython = python3
+deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:bandit]