Add a venv tox job

The docs CI job is failing on trying to run tox -evenv to run build
sphinx. However this job was not defined anywhere. THis commit adds the
job definition to hopefully unbreak the job. To enable the jobs to work
we also need to install the sphinx deps to build the docs. This was
previously done in the tox.ini but this commit moves it to
test-requirements.txt to ensure that all the tox jobs are able to build
docs.

Change-Id: I5457f3e442fe673a81925a8ed2f36f4eab0639cb

test-requirements.txt

@@ -1 +1,6 @@
+pbr>=0.6,!=0.7,<1.0
+Pygments
+docutils
+sphinx>=1.1.2,<1.2
+oslosphinx
 bashate >= 0.2

tox.ini

@@ -8,17 +8,14 @@ install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
 
+[testenv:venv]
+commands = {posargs}
+
 [testenv:bashate]
 commands =
   bash -c "ls *.sh prep-* stop-* upgrade-* from-*/upgrade-* functions | xargs bashate -v {posargs}"
 
 [testenv:docs]
-deps =
-   Pygments
-   docutils
-   sphinx>=1.1.2,<1.2
-   pbr>=0.6,!=0.7,<1.0
-   oslosphinx
 whitelist_externals = bash
 setenv =
   TOP_DIR={toxinidir}