IPv6-primary dual stack

Goal:
Support migration from dual-stack IPv6 to single-stack IPv6.

Why is this important?
We have customers who want to deploy a dual stack cluster and then (eventually) migrate to single stack ipv6 once all of their ipv4 dependencies are eliminated. Currently this isn't possible because we only support ipv4-primary dual stack deployments. However, with the implementation of OPNET-1 we addressed many of the limitations that prevented ipv6-primary, so we need to figure out what remains to make this supported.

At the very least we need to remove the validations in the installer that requires ipv4 to be the primary address. There will also be changes needed in dev-scripts to allow testing (an option to make the v6 subnets and addresses primary, for example).