Update documentation build for new PTI

[Nguyen Hai]

For compliance with the Project Testing Interface as described in:
https://governance.openstack.org/tc/reference/project-testing-interface.html

For more details information, please refer to:
http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/552046

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/552046
Committed: https://git.openstack.org/cgit/openstack/tacker/commit/?id=a205c737fffb81e0ae6346cc02e7bb333027375a
Submitter: Zuul
Branch: master

commit a205c737fffb81e0ae6346cc02e7bb333027375a
Author: Nguyen Hai <email address hidden>
Date: Mon Mar 12 23:59:57 2018 +0900

    Follow the new PTI for document build

    For compliance with the Project Testing Interface as described in:
    [1] https://governance.openstack.org/tc/reference/project-testing-interface.html
    [2] http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html

    Change-Id: I001f8d12a277e520af85ca18a27ff90b3cd98ae2
    Closes-Bug: #1755190