With the recent ceph gate blocker with backups it'd be good to be able
to query for failures in the cinder backup logs, so let's add it to the
logstash index.
Example log: http://goo.gl/4IIWai
Related-Bug: #1476416
Change-Id: Ie5f63e3a4acb5cfef97292cd7c8a009d86ee703a