tox: add a generic environment to build a RST guide

With this environment it is possible to build a single RST guide with
the following common: tox -e build -- NAME

E.g. to build the contributor guide run the following command:

tox -e build -- contributor-guide

Change-Id: I94cf504eece14b2b8d2734cb5b4c16a1b409026f

tox.ini

@@ -68,6 +68,14 @@ commands =
 commands =
   {toxinidir}/tools/build-install-guides-rst.sh ubuntu
 
+[testenv:build]
+commands =
+  {toxinidir}/tools/build-rst.sh \
+    doc/{posargs} \
+    --glossary \
+    --build build \
+    --target {posargs}
+
 [testenv:publishdocs]
 # Prepare all documents (except www subdir) so that they can get
 # published on docs.openstack.org with just copying publish-docs/*