This is needed for publishing in the future:
This is so that we can create a list of directories to ignore when we
rsync documentation builds to a publishing site. Some builds are
published as subdirectories underneath the results of superior
builds.
This allows those superior builds to be rsynced without removing the
other builds that were published under them.
http://specs.openstack.org/openstack-infra/infra-specs/specs/doc-publishing.html
Change-Id: Ic8773caebecafa87b10c1ad4612bde6b1513b667
54aa873505