HOSTNAME is a bash built-in always containing the host's name, so the
"-z HOSTNAME" check never fired and the prompt was silently skipped —
containers ended up named after the Proxmox host.
Also added an optional VLAN tag prompt (empty = no tag), and the network
wait loop now exits with an error if the network never comes up instead
of silently proceeding to a guaranteed-broken apt-get update.
l.kirchner