Add gitlab-sshd configuration support

What does this MR do?

This adds support for enabling gitlab-sshd. gitlab-sshd is currently intertwined with gitlab-shell: the gitlab-shell config file controls gitlab-sshd startup parameters, including the log file and host key settings. /var/log/gitlab/gitlab-shell/gitlab-shell.log actually contains most of the important messages.

Related issues

Relates to #5937 (closed)

Testing

Apply this merge request and add to /etc/gitlab/gitlab.rb:

gitlab_sshd['enable'] = true

Run gitlab-ctl reconfigure.

This should boot gitlab-sshd on port 2222 by default.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened