Update puppet docs to set ROOT_DISK

This is recommended because with puppet /mnt/state is a
waste of space because we aren't actually putting any data there.
With this change the root filesystem will be expanded to consume
all the extra available space on the partition.

Change-Id: I756a164c568809babf6bc10accc3afab30c0d042

doc/source/puppet.rst

@@ -82,6 +82,10 @@ in your environment.
        # Do not manage /etc/hosts via cloud-init
        export DIB_CLOUD_INIT_ETC_HOSTS=''
 
+       # Set ROOT_DISK == NODE_DISK (no ephemeral partition)
+       export ROOT_DISK=40
+       export NODE_DISK=40
+
    By default TripleO uses puppet for configuration only. Packages (RPMs, etc)
    are typically installed at image build time.