Rename 'logs' VG to 'logsbackup'

This was erroneously omitted from
https://github.com/alphagov/govuk_offsitebackups-puppet/commit/1f17a9cd929ab4113e664b88a3996d8cea132fe3
Merge pull request #74 from alphagov/rename-vg

Rename 'logs' VG to 'logsbackup'