fix: filter correct disk image during boot disk selection

[aditsharma55]

Summary

• Fix boot disk image selection that was picking k8s worker-node images (e.g. worker-node-v-1-33-ubuntu24.04-cuda12.8) instead of instance images (e.g. ubuntu24.04-cuda13.0) due to Nebius API pagination and non-deterministic ordering
• Replace the old first-match image selection with a score-based system that evaluates all images across all pages and picks the highest-scored one, ensuring ubuntu24+cuda13 is always preferred over worker-node images for default deployments
• Remove iptables-persistent / netfilter-persistent from cloud-init since the correct instance image (ubuntu24.04-cuda13.0) does not ship with netfilter-persistent, and the previous cloud-init commands were causing failures (sudo: netfilter-persistent: command not found)

What changed

Image selection — pagination fix:

The old code used Image().List() which only returned the first page of results. With a small default page size, ubuntu24.04-cuda13.0 could be omitted entirely. Replaced with Image().Filter() which auto-paginates via the SDK iterator.

Image selection — score-based ranking:

The old if/else matching was order-dependent and first-match-wins. The new approach scores every non-ARM64 image using a tiered system

Cloud-init cleanup:

Removed iptables-persistent package, the netfilter-persistent.service systemd ordering drop-in, and the netfilter-persistent save command. These were added for a previous image that shipped with netfilter-persistent pre-installed; the current image does not have it, and these commands caused cloud-init to fail.

Relevant Linear Ticket:

https://linear.app/nvidia/issue/BRE2-901/issue-when-deploying-nebius-h100