Make the tox docs env build the docs

The tools/build_docs script builds the docs for devstack. Use it.

Change-Id: I530ff6c8090f2c58160cb11bee35534b79db52ed
diff --git a/tox.ini b/tox.ini
index 325adae..3677631 100644
--- a/tox.ini
+++ b/tox.ini
@@ -13,4 +13,11 @@
 commands = bash -c "find {toxinidir} -not -wholename \*.tox/\* -and \( -name \*.sh -or -name \*rc -or -name functions\* -or \( -wholename lib/\* -and -not -name \*.md \) \) -print0 | xargs -0 bashate -v"
 
 [testenv:docs]
-commands = python setup.py build_sphinx
+deps =
+   Pygments
+   docutils
+whitelist_externals = bash
+setenv =
+  TOP_DIR={toxinidir}
+  INSTALL_SHOCCO=true
+commands = bash tools/build_docs.sh