Borgbackup: Accept unknown hosts

This is one way to Fix #118 (closed): Make Borg backup accept new hosts when connecting to our backup service.

Since our backups are encrypted and we do SSH pubkey authentication, I would find it acceptable to allow our backup clients to add unknown IPs to its ssh known-hosts file. With the new setting of StrictHostKeyChecking=accept-new, changed SSH host keys will still be rejected (i.e. it is more secure than StrictHostKeyChecking=no).

More paranoid DevSecOps might want to instead get the backup server's SSH key fingerprints, and distribute them to the storage servers' known-hosts files themselves.