Configure Jaeger for gitlab-shell

What does this Merge Request do and why?

Most of our services use the GITLAB_TRACING envvar to configure Jaeger. However, gitlab-shell uses its config file, since it's difficult to inject environment variables into those processes, being children of the openssh process. This has never been configured to work in GDK.

The persistent gitlab-sshd process could use GITLAB_TRACING from the environment, but doesn't for reasons of consistency.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.