Add a tox target for building documentation

Change-Id: I04cc4d3f5cf4c9d92144fef2a0ed3e3ac248679a
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>

tox.ini

@@ -34,6 +34,12 @@ commands =
     python tools/hacking.py --ignore=E12,E711,E721,E712,N403,N404 --show-source \
         --filename=ceilometer* bin
 
+[testenv:docs]
+deps = -r{toxinidir}/tools/test-requires
+       -r{toxinidir}/tools/pip-requires
+       sphinxcontrib-httpdomain
+commands = python setup.py build_sphinx
+
 [testenv:venv]
 deps = -r{toxinidir}/tools/test-requires
        -r{toxinidir}/tools/pip-requires