fix backup monitoring

As bzillins at HN reported our backup monitoring is broken, which is indeed the case: http://monitor.gitlab.net/dashboard/db/backups?orgId=1

Thank you for heads up sir, this issue is created to track the progress on the monitoring fix.

NOTE: our backups are working, we have all the tar files and wal-e chunks in place (in several, in fact), we're only missing datapoints for the graph.