[#69995468] Add another disk to Vagrant

Adding another disk for log backups to use.
[#69995468] Add a user for log backups.

Add a second user to receive transition log backups.
[#69995468] Add and mount a LVM volume for log backups.
[#69995468] Allow sftp for rssh users.

Duplicity seems to use `sftp` instead of `scp` even if you tell it to use
`scp` as a protocol

I would have preferred to use `sshd`'s internal `sftp` subsystem but the
puppet module we are using doesn't easily allow that change.
Merge pull request #42 from alphagov/69995468_add_disk_for_log_backups

[#69995468] Set up offsite backup machine for transition log backups.

Self merging since  @jennyd doesn't have access.