Add doc/requirements

We need to specify doc requirements in doc/requirements.txt
to avoid problems with the pip resolver for the release team.
Removed specific doc requirements from test-requirements.txt

Change-Id: Ia88848fd9c4ce35d396bc186ae3894a1d5865db9
diff --git a/tox.ini b/tox.ini
index 3c18fbf..d3e722a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -39,9 +39,12 @@
     coverage xml -o cover/coverage.xml
 
 [testenv:docs]
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+deps = {[testenv:docs]deps}
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html