Replaces the template-clone flow with: create empty VM shell, download the
cloud image on the node (cached by filename, optional checksum verification),
qm importdisk, attach as scsi0. NIC config, snippet upload, ssh keys, disk
resize, and start remain unchanged (already generic).
New helpers: _run_node_command (strict SSH exec with custom timeout and
non-zero-exit detection, unlike the best-effort _exec_ssh_command),
_download_cloud_image (idempotent download + checksum check),
_find_default_image_storage (content=images discovery, mirrors the existing
snippet-storage discovery).
24 tests pass (10 new: _run_node_command x2, _download_cloud_image x4, plus
rewrites of the 4 existing create_vm_from_cloud_init tests for the new flow).
Christian Manivong
9264cdcba9