Configure backup host and setup nas01 as a backup host

This works by sshing into the servers to be backed up as the relevant user where
authorized_keys causes the correct command to run to spit the backup back
down the incoming ssh connection. This way the backup host can only take
backups and the backed up server has no permissions on the backup host
String interpolation can't use variables concurrently defined

So don't do that then. Write a few more lines that do the same thing in a slightly clunky way.