Setup of LXD tests needs updating for artful & bionic

[Rod Smith]

Through at least xenial, the LXD images on http://cloud-images.ubuntu.com included a file with a name taking the form of {release}-server-cloudimg-{arch}-root.tar.xz, and this filename/URL is used by both the maniacs-setup and curtin_userdata preseed scripts. Beginning with artful (or maybe earlier), though, this file has been eliminated. It appears that it's been replaced by {release}-server-cloudimg-{arch}.squashfs, so that should be used instead, at least for artful and later. If this change is not made, the precheck script will report that LXD hasn't been properly configured; and depending on what's done manually with configuration, the LXD test in Checkbox may fail.