Dual Stack (add IPv6) Support for BOSH Bootloader (AWS)

[peanball]

In the frame of RFC0038, we would like to extend bosh-bootloader with IPv6 to support dual stack networking.

This change starts with the Terraform scripts and enables the changes in BOSH to support dual stack networking.

Tasks:

• Adapt the implementation in Terraform
• Ensure that new deployments can be created IPv4 or dual stack
• Allow existing deployments to be updated to dual stack from IPv4

A migration path for existing bbl deployments could also be something that is added via additional change later.

Details:

• feat: Add IPv6 support to IP and CIDRBlock #633
  Extend or remove IP and CIDRBlock types. They are hardcoded to support only IPv4. They could be extended using Go's native net.IP, net.IPMask, etc. to do the same thing but across IPv4 and IPv6.
• feat: Add dualstack support for AWS Infrastructure #640
  Enable IPv6 for AWS on terraform via the TF variable dualstack
• Configure BOSH via generated ops files for dual stack, when dualstack output variable is true.
  • Depends on Introduce prefix allocation bosh#2611
• Add the NLB load balancer type variant for AWS, as that one is needed for IPv6 ingress. This NLB should also do TLS termination, like cf_lb already does on AWS.

[peanball]

@rkoster mentioned in the Foundational Infrastructure meeting today that deployment updates should not be in scope, at least not for the first implementation. bosh-bootloader is primarily used for test environments.

Migration could be contributed later, possibly also by other parties as needed.