Add support for explicit ipv6 features

[brett19]

Broadly speaking, I think that cbdinocluster already supports some variant of IPv6, since the containers used by the docker deployer will be assigned IPv6 addresses. We don't however currently expose a mechanism to fetch those IPv6 addresses via the connstr or ips commands. Additionally, we do not support configuring the cluster in IPv4-strict or IPv6-strict modes, which may be required for some forms of testing. I believe there are a number of places that we assume IPv4 addresses when doing the initial cluster setup as well.