use headscale server url as domain instead of base_domain

[kradalby]

Fixes #2336
Closes #2344
Closes #2318

[valkum]

Possible duplicate of #2344 (had the work lying around but didn't create a PR yet). I think my PR is more complete, as the DNS base domain is completely separate from the org base domain in tailscale.

[kradalby]

I was just going to post on yours, I do not think it should be based on the base domain at all, so I do not think introducing another layer of config and falling back to the wrong one is the ideal setup.

Since Headscale support one tailnet, I think using the name of the server makes more sense. the naming of base_name is already confusing enough and introducing a second one would not help I believe.

[valkum]

The fallback is just for a transitional period. It can be removed in an upcoming version.
I want to use different values. e.g. hs.my-domain.com as the headscale hostname and my-domain.com as the org domain, but the magic DNS base domain should be my-domain.net.

[kradalby]

the problem isnt the fallback, its adding more configuration options, we are increasingly getting more strict as there are has previously been added two many knobs making it hard for us to move forward without making anger. In this case we will continue to go for less configurability and defaults based on what we think are best practices.