Align nailgun docs building with OS-Infra

This is a first step in moving Nailgun API docs building and publishing
to upstream infra. To enable gate-fuel-web-docs job docs should be able
to be built with 'tox -evenv -- python setup.py build_sphinx' command and
html output should be available in doc/build/html directory.

Change-Id: I42eb69a4060f50c8ef99cef4d54577078c1c846c

nailgun/setup.cfg

@@ -0,0 +1,4 @@
+[build_sphinx]
+all_files = 1
+build-dir = ../doc/build
+source-dir = ../docs

nailgun/test-requirements.txt

@@ -1,4 +1,5 @@
 -r requirements.txt
+docutils>=0.11
 hacking==0.10
 mock==1.0.1
 nose==1.1.2

tox.ini

@@ -77,7 +77,7 @@ commands =
 setenv = NOSE_WITH_COVERAGE=1
 
 [testenv:venv]
-deps = -r{toxinidir}/nailgun/requirements.txt
+deps = -r{toxinidir}/nailgun/test-requirements.txt
 commands = {posargs:}
 
 [testenv:pep8]