spread(lxd): work around SSH password authentication disabled by livecd-rootfs

[bboozzoo]

In recent images, livecd-rootfs may be disabling SSH password authentication by default, with the intention that cloud-init would enabled that by placing a higher priority file under /etc/sshd/sshd_config.d/. See https://git.launchpad.net/livecd-rootfs/commit/live-build/ubuntu-cpc/hooks.d/chroot/052-ssh_authentication.chroot?id=480d5b26ea97e0bfebac8aedfa1f2bf7286f027a for reference.

Do the same as cloud-init would a drop a higher priority config overriding relevant SSHD settings.

[bboozzoo]

I'll extend the lxd backend spread test to cover more recent images.

[bboozzoo]

LXD backend spread tests have been updated with Ubuntu 18.04, 20.04, and 22.04.

[bboozzoo]

Looks like a similar fix but done slightly differently was proposed in #179

[thp-canonical]

Also related, a similar fix for the Google backend: #155