Correct bup-excludes file installation.

* modules/bup/manifests/init.pp: The /etc/bup-excludes file was
being generated incorrectly, making it useless for its intended
purpose. It was also misspelled.

Change-Id: Ia279aab88744e7b346bcc350bcfc106f08b2e191
Reviewed-on: https://review.openstack.org/20095
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins