pebble-challtestsrv: allow defaultIPv4 to be a hostname

[mitar]

Currently one has to provide just an IP address for defaultIPv4. But when running inside CI you might not know the IP because it could be dynamic, but you do get a hostname: e.g., in Docker network setting a target container might get a hostname you can resolve to. Being able to provide that hostname directly instead of having to script resolving it first myself would simplify things a lot.

My context is GitLab CI where I can define a service but the command is a hard-coded string.

My current workaround is to provide an ash command-line script which resolves the IP and then calls pebble-challtestsrv, but it is ugly. (An alternative to put that into a script would instead require from me to build a custom Docker image.)

[mitar]

I made letsencrypt/challtestsrv#20.