support CHATMAIL_SERVER environment variable in generate-dns-zone.sh

[missytake]

In https://github.com/deltachat/chatmail/blob/main/deploy-chatmail/src/deploy_chatmail/deploy.py#L8, we support to specify CHATMAIL_SERVER explicitly, if you want it to be different than CHATMAIL_DOMAIN.

The use case for this is if you already have a website for example.org, and now want to deploy a chatmail server offering @example.org mail accounts on a different server, these values need to point to different machines.

We should setup a test server with this use case to test this scenario.

[hpk42]

I remain a bit skeptical it's worth at this point to support a separation model for mx and http servers. I like the simplicity of the "one machine model" . But i think you and Alex are more in favor of it so ok.
But Let's at least use CHATMAIL_HTTP here .... "Server" is pretty ambiguous in my reading. We already use CHATMAIL_SSH.

[link2xt]

Is there are request for this, e.g. do you want to run this setup yourself?

How would the mail server be discovered when user is configuring DC? Don't you need an autoconfig subdomain pointing to the chatmail server then?

[link2xt]

I don't have any need for this myself, guess @missytake wants to run it like this for some existing domain.

The usecase as I understand is similar to running a chatmail server for delta.chat domain without the need to give control of https://delta.chat to the chatmail server.

[missytake]

I don't have a pressing need either to be honest, just the expectation that it might be a matter for other organizations who'd be interested in hosting a chatmail server. Let's close this PR for now and revive it later if the need arises.