Import docs building job

This repo had a docs building job setup in project-config repo and was
at one time publishing to readthedocs. The publishing needs a manual
change and the repo was never adapted.

Import just the docs building jobs using the build-openstack-docs-pti
template.
If the repo wants to use publish later to readthedocs, the template
can be changed to docs-on-readthedocs as explained in
http://lists.openstack.org/pipermail/openstack-dev/2018-August/132836.html

Fix tox -e docs building: Add doc/requirements.txt file, include doc8,
use python3 for docs building.

Change-Id: Ie916347b783d9d016f3e8e95c66676d6fe7da8a4

.zuul.yaml

@@ -16,6 +16,7 @@
 
 - project:
     templates:
+      - build-openstack-docs-pti
       - windmill-jobs-fedora-latest
       - windmill-jobs-bionic
     check:

doc/requirements.txt

@@ -0,0 +1,2 @@
+sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
+doc8>=0.6.0 # Apache-2.0

tox.ini

@@ -8,6 +8,7 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 whitelist_externals =
   bash