Add ipv6_address, ipv6_network, ipv6_netsize

[ghost]

Example:

  provisioner "local-exec" {
    command = "echo ${join(", ", vultr_server.gw_hosts.*.ipv6_address)}"
  }

This only picks the first IPv6 address and disregards all others, but that's good enough for a start. It's also in line with the IPv4 address behaviour.

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(The indentation change around snapshot_id is from go fmt which my editor runs automatically. Let me know if I should remove it.)

[rtizzy]

Howdy @lgierth! Thanks a lot for taking the time to add IPv6 support to this plugin.

If you'll update the example.tf file to properly document this new feature, then I'll merge your pull.

Thanks again!

[rtizzy]

@lgierth

Howdy. Do you have a moment to add the documentation for this feature? If so, I'd greatly appreciate it.

If not, I can see about integrating it myself.