Backups: add necessary configuration to backup into Gohma over rsync.

For our servers that do backups into Gohma, the backup playbook will now
configure both that server and Gohma itself to let backups happen. It will
ensure the source server has a ssh key, create the backup account in Gohma,
and authorize that ssh key.

It seems the backup account is {host}backup with home directory
BACKUP.{host}.kde.org in all cases, but for flexibility I'm putting that
in host variables.