Follow the new PTI for document build

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

Change-Id: I84e614346cfa4d302f919d12bea9680c5e527475

The problem is not complicated, just a few steps
1. Move the document-related from test-requirements to doc/requirements
2. Documentation testing use doc/requirements to install requirements package.

https://review.openstack.org/#/c/529006/
I have an example in cinder, can refer to (if no one wants to do within two days, I did it myself ...)