Add docs to python-barbicanclient gates

We would like to add python-barbicanclient-docs to the check and gate
pipelines to be able to review the output of document changes in the
same manner that the Barbican server changes are reviewed.

Document changes should still be published by
openstack-client-publish-jobs upon release.

Change-Id: If02ac195d344e223ea509431c58a4d01e3b9f5dd
Merge "Add docs to python-barbicanclient gates"