feat(vm-provision): let Proxmox download cloud images into an import storage

Provisioning downloaded every cloud image over SSH into
/var/lib/vz/template/netork-images, on the node's root filesystem. On a
small root that fills up and takes Proxmox down with it (netOrk #480).

When the node has an active storage with content type "import" (Proxmox
8.2+), Proxmox now does it itself: download-url with checksum
verification into that storage, then import-from as the root disk. The
file is named after a hash of the full URL and reused when present.
Proxmox takes the format from the extension and has no ".img", so
Ubuntu's qcow2 .img is stored as .qcow2 -- a wrong guess fails at import
instead of attaching a qcow2 container as a raw disk.

Without an import storage, or for an image type Proxmox cannot import,
the SSH download is used as before.

CHANGELOG.md

@@ -8,6 +8,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- Cloud images for `create_vm_from_cloud_init` are downloaded by Proxmox itself
+  when the node has an active storage with content type `import` (Proxmox
+  8.2+): `download-url` with checksum verification, then `import-from` as the
+  root disk. The image no longer passes through the node's root filesystem.
+  Files are named `netork-<url hash>-<name>.<qcow2|raw|vmdk>` and reused;
+  Ubuntu's `.img` is stored as `.qcow2`. Without such a storage, or for an
+  image type Proxmox cannot import, the SSH download into
+  `/var/lib/vz/template/netork-images` is used as before.
 - `HypervisorDriver` contract methods `start_vm`, `stop_vm`, `reboot_vm`,
   `suspend_vm` and `get_vm_config`. They accept a VM's name or vmid, raise
   `ValueError`/`RuntimeError` instead of returning a result dict, and wait