Follow the new PTI for document build

For compliance with the Project Testing Interface as described in:

https://governance.openstack.org/tc/reference/project-testing-interface.html

For more detials information, please refer to:

http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html

Change-Id: I32b98aea4b2720a527c0e436eaf1e6874df1a967

doc/requirements.txt

@@ -0,0 +1,7 @@
+# The order of packages is significant, because pip processes them in the order
+# of appearance. Changing the order has an impact on the overall integration
+# process, which may cause wedges in the gate later.
+# this is required for the docs build jobs
+sphinx!=1.6.6,>=1.6.2 # BSD
+openstackdocstheme>=1.18.1 # Apache-2.0
+reno>=2.5.0 # Apache-2.0

tox.ini

@@ -15,11 +15,14 @@ deps = -r{toxinidir}/requirements.txt
 commands = {posargs}
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+  -r{toxinidir}/doc/requirements.tx
+commands = sphinx-build -b html doc/source doc/build/html
 
 [testenv:spelling]
 deps =
    -r{toxinidir}/requirements.txt
    sphinxcontrib-spelling
    PyEnchant
-commands = sphinx-build -b spelling doc/source doc/build/spelling
+commands = sphinx-build -b spelling doc/source doc/build/spelling