Fix doc builds

The updated releasenote and requirements jobs expect requirements for
docs in one of the requirements files. Move requirements from setup.cfg
to doc/requirements.txt file.

See also
https://governance.openstack.org/tc/reference/project-testing-interface.html#documentation

Change-Id: Ic454831a80f799c2be5ba3089001f31f68c39f85

doc/requirements.txt

@@ -0,0 +1,2 @@
+sphinx>=1.6.2 # BSD
+openstackdocstheme>=1.11.0 # Apache-2.0

setup.cfg

@@ -33,9 +33,6 @@ test =
     pifpaf[gnocchi]>=0.23
     gnocchi[postgresql,file]
     mock>=1.2  # BSD
-doc =
-    sphinx>=1.6.2 # BSD
-    openstackdocstheme>=1.11.0 # Apache-2.0   
 
 [entry_points]
 console_scripts =

tox.ini

@@ -24,7 +24,7 @@ commands = flake8
 
 [testenv:venv]
 # Used by openstack-infra to build the doc
-deps = .[doc]
+deps = -r{toxinidir}/doc/requirements.txt
 commands = {posargs}
 
 [testenv:cover]
@@ -36,7 +36,7 @@ commands =
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:docs]
-deps = .[doc]
+deps = -r{toxinidir}/doc/requirements.txt
 commands =
    rm -rf doc/source/ref
    python setup.py build_sphinx