Changes:
1. Add MAX_POLL_TIMEOUT env var for large Proxmox clusters that need
more than 3 minutes for polling (default: 3m, minimum: 30s)
2. Handle external Ceph storage gracefully - don't mark nodes unhealthy
when Proxmox returns 'binary not installed' (e.g., for Ceph not
managed by Proxmox)
Related to #965