Change mapper/data-backup to mapper/backup-data

We misunderstood the way our LVM volumes derive their name in
/dev/mapper which has been causing provisioning failures - see
https://deploy.preview.performance.service.gov.uk/job/puppet-deploy/449/console

This should fix them.
Merge pull request #328 from alphagov/fix-lvm-dev-mapper

Change mapper/data-backup to mapper/backup-data