make cidr/netmask optional for IPv6 subnets

[evgeni]

Previously, the API required the mask parameter and we either passed what the user entered or calculated it from cidr. The API never documented the cidr param so we never sent it. Now it does and so we send both. That's fine.

However, now both mask and cidr are optional, as they are only required for IPv4 subnets, but not for v6 ones. We still have required_one_of=[mask,cidr] -- there is no required_one_of_if=['type=IPv4', [mask,cidr] so we would need to wrap that somehow ourself, or keep mask/cidr required for now, which is what I landed on.

Originally posted by @evgeni in #876 (comment)