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: I3f7a780a10b95d1ab1fb28d05fc15d8022591e0d

doc/requirements.txt

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

setup.cfg

@@ -36,9 +36,6 @@ test =
     requests-mock>=1.1 # Apache-2.0
     testrepository>=0.0.18 # Apache-2.0/BSD
     testscenarios>=0.4 # Apache-2.0/BSD
-doc =
-    sphinx>=1.6.2  # BSD
-    openstackdocstheme>=1.11.0 # Apache-2.0
 
 [entry_points]
 console_scripts =

tox.ini

@@ -16,7 +16,7 @@ commands = flake8 {posargs}
 
 [testenv:venv]
 # Used by openstack-infra to build the doc
-deps = .[doc]
+deps = -r{toxinidir}/doc/requirements.txt
 commands = {posargs}
 
 [testenv:cover]
@@ -25,7 +25,7 @@ commands =
   coverage report
 
 [testenv:docs]
-deps = .[doc]
+deps = -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf doc/source/ref
   python setup.py build_sphinx