Make it possible to disable OpenSSH server in Docker image

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What does this MR do?

Make it possible to disable OpenSSH server in Docker image

Includes a simple heuristic if OpenSSH is needed.

I have moved the whole OpenSSH setup after the gitlab-ctl reconfigure, so we can inspect the generated setup (which services will be started) and base our initial decision if OpenSSH should be started on that. The heuristic can be skipped by setting the GITLAB_DISABLE_OPENSSH environment variable.

Tested with different configurations modeled after the 2k reference architecture to validate that the OpenSSH server is only started in the rails container and none other.

PS: I took the environment variable name from a suggestion in issue #9156, but it might be better to invert the name to avoid double-negation? For example, GITLAB_ENABLE_OPENSSH with a default value of "true"?

Related issues

First stab at implementing #9156 to get the discussion going.

Checklist

See Definition of done.

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Expected (please provide an explanation if not completing)

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