Update docs job

Replace obsolete sphinx-build job with tox-docs job, use
a template for this.

The newer job uses "tox -e docs" directly.

Add sphinx requirements for python 3.

Change-Id: I1acc55a470ebacce812ba6840663302d759f69b7

test-requirements.txt

@@ -2,6 +2,8 @@ hacking>=1.1.0,<1.2.0
 coverage>=4.0
 fixtures>=3.0.0
 python-subunit>=1.0.0
+sphinx>=1.6.5,<2.0.0;python_version=='2.7'  # BSD
+sphinx>=1.6.5;python_version>='3.4'  # BSD
 openstackdocstheme>=1.18.1 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
 testscenarios>=0.4

zuul.d/project.yaml

@@ -1,8 +1,3 @@
 - project:
-    check:
-      jobs:
-        - build-openstack-sphinx-docs
-
-    gate:
-      jobs:
-        - build-openstack-sphinx-docs
+    templates:
+      - build-openstack-docs-pti