Error out on warnings when building RST

Treat warnings as errors when building RST.

Fix warning about wrong reference in sdk files and about
file not included anywhere.

Change-Id: I3cfd054b6bacc205991e2f57795d183a21275c21

doc/playground-user-guide/source/sdk.rst

@@ -24,4 +24,4 @@ You should also be familiar with:
    sdk_configure_access_security_instances.rst
    sdk_authenticate_networking_endpoint.rst
    sdk_authenticate_compute_output.rst
-.. add sdk_assign_cors_headers.rst when migrated TODO(DC)
+   sdk_assign_cors_headers.rst

doc/playground-user-guide/source/sdk_authenticate.rst

@@ -1,3 +1,5 @@
+.. _sdk_authenticate:
+
 ============
 Authenticate
 ============

doc/playground-user-guide/source/sdk_authenticate_compute_output.rst

@@ -9,7 +9,7 @@ Set environment variables
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 To set up environmental variables and authenticate against Compute API
-endpoints, see :ref:`Authenticate <sdk_authenticate.rst>`.
+endpoints, see :ref:`Authenticate <sdk_authenticate>`.
 
 .. _get-openstack-credentials:
 

tox.ini

@@ -45,10 +45,10 @@ commands =
   python tools/www-generator.py --source-directory www/ --output-directory publish-docs/www/
   cp www/www-index.html publish-docs/
   rsync -a www/static/ publish-docs/www/
-  sphinx-build doc/playground-user-guide/source/ doc/playground-user-guide/build/html
+  sphinx-build -W doc/playground-user-guide/source/ doc/playground-user-guide/build/html
   mkdir -p publish-docs/playground-user-guide/content/
   rsync -a doc/playground-user-guide/build/html/ publish-docs/playground-user-guide/content/
-  sphinx-build doc/networking-guide/ doc/networking-guide/build/html
+  sphinx-build -W doc/networking-guide/ doc/networking-guide/build/html
   mkdir -p publish-docs/networking-guide/content/
   rsync -a doc/networking-guide/build/html/ publish-docs/networking-guide/content/
   # Do not build DocBook XML Networking Guide
@@ -56,11 +56,11 @@ commands =
 
 [testenv:docs]
 commands = 
-  sphinx-build doc/playground-user-guide/source/ doc/playground-user-guide/build/html
-  sphinx-build doc/networking-guide/ doc/networking-guide/build/html
+  sphinx-build -W doc/playground-user-guide/source/ doc/playground-user-guide/build/html
+  sphinx-build -W doc/networking-guide/ doc/networking-guide/build/html
 
 [testenv:network]
-commands = sphinx-build doc/networking-guide/ doc/networking-guide/build/html
+commands = sphinx-build -W doc/networking-guide/ doc/networking-guide/build/html
 
 [testenv:publishdocs]
 # Prepare all documents (except www subdir) so that they can get