Docs build: Treat warning as error

Let's treat warnings as errors. Typical warnings are not
existing link targets, missing toctree entry,...

Change-Id: Ic7b70026ef508cd0c41be4fc35435bc15e429916

doc/source/getting_started.rst

@@ -49,8 +49,8 @@ The following actions are done for the compute node.
       assign to a single instance?
 
       .. note::
-      OpenStack does *not* use those processors exclusively. These processors
-      are still shared with non-OpenStack partitions.
+        OpenStack does *not* use those processors exclusively. These processors
+        are still shared with non-OpenStack partitions.
 
     * What is the maximum amount of memory that can be consumed by all
       OpenStack instances on this CEC?

doc/source/image/rhel7/rhel7_image.rst

@@ -83,7 +83,7 @@ This package is required for growing the filesystem.
 
 * install it::
 
-  rpm -i cloud-utils-growpart-0.27-9.fc20.noarch.rpm
+    rpm -i cloud-utils-growpart-0.27-9.fc20.noarch.rpm
 
 
 Add the RHEL7.3 yum repository

setup.cfg

@@ -28,6 +28,7 @@ packages =
 source-dir = doc/source
 build-dir = doc/build
 all_files = 1
+warning-is-error = 1
 
 [upload_sphinx]
 upload-dir = doc/build/html