Add pre-install user data

[nickpetrovic]

I've been looking at adding NAT64 support to this and have been successful using the nat_instance_user_data_post_install variable. However, I noticed that when an instance is running in a dual stack network, DNS resolution is extremely slow (or possibly just API calls in general). For example, it takes 15m to provision an alternat instance + my post install script. I discovered it was the awscli calls that took forever.

I found a way to increase the precedence of IPv4 addresses on the host using the getaddrinfo config. To test it, I wanted to make sure it was the first thing that happened in the user data, so I added a nat_instance_user_data_pre_install variable and set it to this:

nat_instance_user_data_pre_install = <<-EOF
  # Prefer IPv4 over IPv6
  echo 'precedence ::ffff:0:0/96 100' >> /etc/gai.conf
EOF

My instance setup time in a dual stack network went from ~15m to 2m.

Since I don't want to make any assumptions about people's networks (route table and subnet configurations) or whether they're using Jool vs Tayga (NAT64), I don't think it would make sense to add any of that stuff to this project, but a pre-install user data variable might?

[nickpetrovic]

Just added an ipv6-specific ingress variable to allow ipv6 ingress so that the internal network can communicate with the nat instance over ipv6 in a dual stack network.

[bwhaley]

Nice, thanks for this!

One small ask: could you add a note in README.md (in the Other Considerations section) about how to use this variable to set IPv4 precedence? Might save some time for others who may not search and find this PR.

[nickpetrovic]

@bwhaley Added.

[bwhaley]

Thank you for the contribution.