Add doc/requirements.txt file with doc requirements

The sphinx jobs need to find doc requirements in either
test-requiremnts.txt or doc/requirements.txt. Putting them directly in
to tox.ini, not so much.

Change-Id: I98a43b511a6949fa4f00c26eec224d24d6fa6588
(cherry picked from commit 4187d2fc4a)

doc/requirements.txt

@@ -0,0 +1,10 @@
+pbr>=2.0.0,!=2.1.0
+
+Pygments
+docutils
+sphinx>=1.6.2
+openstackdocstheme>=1.11.0
+nwdiag
+blockdiag
+sphinxcontrib-blockdiag
+sphinxcontrib-nwdiag

tox.ini

@@ -34,16 +34,7 @@ commands = bash -c "find {toxinidir}             \
          -print0 | xargs -0 bashate -v -iE006 -eE005,E042"
 
 [testenv:docs]
-deps =
-   Pygments
-   docutils
-   sphinx>=1.6.2
-   pbr>=2.0.0,!=2.1.0
-   openstackdocstheme>=1.11.0
-   nwdiag
-   blockdiag
-   sphinxcontrib-blockdiag
-   sphinxcontrib-nwdiag
+deps = -r{toxinidir}/doc/requirements.txt
 whitelist_externals = bash
 setenv =
   TOP_DIR={toxinidir}
@@ -51,11 +42,5 @@ commands =
   python setup.py build_sphinx
 
 [testenv:venv]
-deps =
-   pbr>=2.0.0,!=2.1.0
-   sphinx>=1.6.2
-   openstackdocstheme>=1.11.0
-   blockdiag
-   sphinxcontrib-blockdiag
-   sphinxcontrib-nwdiag
+deps = -r{toxinidir}/doc/requirements.txt
 commands = {posargs}