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

Refer to:
http://lists.openstack.org/pipermail/openstack-dev/2017-November/124815.html

Change-Id: I700b2fb991d2fa2abc1d8ec7221c1470e7634b8e

tox.ini

@@ -7,5 +7,5 @@ envlist = releasenotes
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 
 [testenv:releasenotes]
-deps = -r{toxinidir}/test-requirements.txt
+deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html