fix(proxmox): set guest DNS via cloud-init resolv_conf in vendor snippet

[v0l]

Problem

Customers on the Alpine image (and other minimal images) booted with an empty /etc/resolv.conf and no working DNS.

Root cause

LNVPS assigns IPs statically via Proxmox cloud-init (ipconfig0, no DHCP). When no explicit nameserver is set, Proxmox falls back to the host's /etc/resolv.conf and hands those servers to the guest's cloud-init network config — for every image. The failure is entirely guest-side rendering:

• Ubuntu/Debian/CentOS render cloud-init DNS through systemd-resolved / resolvconf, so /etc/resolv.conf gets populated (Ubuntu also has systemd-resolved fallback DNS masking the issue).
• Alpine uses busybox ifupdown with cloud-init's ENI renderer. It writes dns-nameservers into /etc/network/interfaces.d/50-cloud-init, which only reaches /etc/resolv.conf if openresolv/resolvconf is installed — the stock Alpine cloud image doesn't ship it. So the DNS it was handed goes nowhere.

Just setting --nameserver explicitly would not fix this, since the gap is in how Alpine applies whatever DNS it receives.

Fix

Extend the shared cloud-init vendor snippet (ensure_vendor_snippet) to force a fixed set of public resolvers into every guest:

#cloud-config
ssh_deletekeys: false
manage_resolv_conf: true
resolv_conf:
  nameservers:
    - 1.1.1.1
    - 8.8.8.8
    - 9.9.9.9
    - 2606:4700:4700::1111
    - 2001:4860:4860::8888
    - 2620:fe::fe

cloud-init's resolv_conf module writes /etc/resolv.conf directly, bypassing the guest network renderer. Fixes Alpine, NixOS, and any minimal image in one uniform change. The resolver list (Cloudflare, Google, Quad9 — IPv4 + IPv6) is a fixed constant; no host SSH read is performed.

Adds pure helper build_vendor_snippet with unit tests.

Caveats

1. cloud-init's resolv_conf module runs per-instance (once), so this fixes newly provisioned VMs and reinstalls. Existing affected VMs need a reinstall (or manual cloud-init clean / resolv.conf edit).
2. systemd-resolved distros (Ubuntu) will have their stub /etc/resolv.conf replaced with a direct file — benign for a VPS.

Tests

• test_build_vendor_snippet — empty list, an IPv4+IPv6 pair, and the production constant (asserts both IPv4 and IPv6 entries)

All host::proxmox tests pass.