For compliance with the Project Testing Interface as described in:
https://governance.openstack.org/tc/reference/project-testing-interface.html
For more detials information, please refer to:
http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html
To handle this, this commit changes:
- Introduce doc/requirements.txt
- Update tox.ini [docs] target for developer convenience
- Fixes a lot of warnings caused by a newer sphinx 1.6.5 because
sphinx specified in upper-constraints.txt is used in the new PTI.
- Drop unnecessary [pbrp] warnerrors in favor of warning-is-error.
Change-Id: If40305044c9dfe0024b64bd3921232bb0a6c9372