Fix to flags configuring ip addresses for ipv4+ipv6 compatebility

[consideRatio]

Small changes to help z2jh avoid IPv4 and/or IPv6 issues by not specifying either :: or 0.0.0.0 but instead a blank string when we want to listen far and wide as we do in a z2jh container.

Review notes

I'd love to have this merged soon and cut a release of CHP to bump if for z2jh 1.1.0 that I'd like to make next week.

Related

• Proxy pod fail to start when IPv6 is disabled (listen EAFNOSUPPORT: address family not supported) zero-to-jupyterhub-k8s#2254
• Allow CHP to function in a IPv4 only and/or IPv6 only context zero-to-jupyterhub-k8s#2318

[consideRatio]

Note that these were defaults that were set above during CLI parsing anyhow. By changing this, the actual change in logic outcomes are when we pass a blank string and args.apiIp || "localhost" no longer overrides the falsy blank string.

[consideRatio]

I've verified that CHP became accessible when configured with a blank string just as if it would have been configured with 0.0.0.0 via Z2JH's test suite, but I think using a blank string, we also acquire the sensible IPv4 and/or IPv6 compatibility we want.

[consideRatio]

Script to validate CLI parsing logic relevant to this PR

#!/usr/bin/env node

var cli = require("commander");
var log = require("../lib/log.js").defaultLogger();

var args = cli
  .option("--ip <ip-address>", "description...")
  .option("--api-ip <ip-address>", "description...", "localhost")
  .option("--metrics-ip <ip-address>", "description...", "")
  .parse(process.argv)
  .opts();

if (args.ip == "") {
    log.error("--ip was a blank string");
} else {
    log.error("--ip was: " + args.ip);
}

if (args.apiIp == "") {
    log.error("--api-ip was a blank string");
} else {
    log.error("--api-ip was: " + args.apiIp);
}

if (args.metricsIp == "") {
    log.error("--metrics-ip was a blank string");
} else {
    log.error("--metrics-ip was: " + args.metricsIp);
}