Add test documentation for docs tox target

Enable sphinx to generate documentation from docstrings by running
'tox -e docs'.

Change-Id: I5996e5f07493f69f14172b4bb0535852e89d5456
diff --git a/tox.ini b/tox.ini
index c2fc078..2f7da5c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -40,8 +40,15 @@
 
 [testenv:docs]
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -W -b html doc/source doc/build/html
+commands =
+  rm -rf doc/source/tests
+  sphinx-apidoc -f -o doc/source/tests neutron_tempest_plugin
+  rm -rf doc/build
+  sphinx-build -W -b html doc/source doc/build/html
+allowlist_externals =
+    rm
 
 [testenv:releasenotes]
 deps = {[testenv:docs]deps}