Use openstack theme for docs

This commit makes the doc use the openstack theme, and also adds
build-openstack-docs-pti job to ensure the tox job.

Change-Id: Iffa590d8325285094cd1d0bef9c5850616c9dedc

.zuul.yaml

@@ -3,3 +3,4 @@
       - openstack-python-jobs
       - nodejs4-jobs
       - openstack-python36-jobs
+      - build-openstack-docs-pti

doc/source/conf.py

@@ -36,7 +36,7 @@ master_doc = 'index'
 
 # General information about the project.
 project = u'openstack-health'
-copyright = u'2015, OpenStack Foundation'
+copyright = u'2015-2019, OpenStack Foundation'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
@@ -77,7 +77,7 @@ modindex_common_prefix = ['openstack_health.']
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'nature'
+html_theme = 'openstackdocs'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the