scp from lxd units will fail because hostname is None

[Joe Guo]

For lxd units, backup will fail with:

"error_reason": "command failed: ['scp', '-i', '/var/lib/jujubackupall/ssh/juju_id_rsa', '-o', 'StrictHostKeyChecking=no', '-q', '-B', 'ubuntu@None:/home/ubuntu/etcd-snapshots/etcd-snapshot-2021-12-28-16.13.13.tar.gz', '/mnt/juju-backup/site-manual-juju-controller/k8s-prod/etcd']"

The root issue is caused by `machine.dns_name` in python-libjuju[0][1], which returns None.

However, before upstream is fixed (which would be slow and not in our control), we can add a patch in this repo to workaround the issue.

[0]: https://github.com/juju/python-libjuju/blob/master/juju/machine.py#L231
[1]: https://github.com/juju/python-libjuju/issues/611