Overcloud upgrade jobs often fail when running on nodes with a 38G root
disk. The overcloud upgrade itself generally works, but post-upgrade
testing fails, for example when RabbitMQ gets in alarm state due to lack
of free disk space, which prevents Nova from functioning correctly and
makes it return "Unknown Error (HTTP 504)".

Change-Id: I536a526340f6d7dea7a69372904b777292774eba