Add branch-tarball job for JJB.

The jenkins-job-builder-branch-tarball job was listed in zuul's
layout.yaml, but the template was not expanded in the JJB
projects.yaml for it.

Change-Id: Ie20b490fa5a96f82e6c259c3032224ec323d7627
Reviewed-on: https://review.openstack.org/26037
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins