Provisioning over IPv6 not possible

Currently provisioning instances via IPv6 is not possible. I encountered the following issues:

1. No IPv6 address is assigned to the EC2 instance (in case IPv6 subnets exist) and there is no way to force it.
2. The plugin by default creates a security group rule to allow SSH only for `0.0.0.0/0`. It needs to create the same rule for `::/0` for IPv6, too.
3. You can also not explicitly force this by setting `temporary_security_group_source_cidrs` to `::/0`, because that configuration option does not support IPv6 prefixes **which is not documented**.
4. The plugin never tries to connect to SSH via IPv6 if available. When `ssh_interface` is set to `public_ip` (default) or `private_ip` it should choose the assigned IPv6 address first.

This makes it impossible to use this plugin in IPv6-only environments.