Make the router use port 3000 by default

What does this merge request do and why?

Related issue: gitlab#450958 (closed)

Make the router use port 3000 by default

This saves us having to reconfigure gitlab/config/gitlab.yml to use a different port: value.

This setup mirrors how GitLab.com will be setup soon.

How to set up and validate locally

1. Set up a local interface
2. Run gdk config set gitlab_http_router.enabled true
3. Run gdk reconfigure
4. Run gdk restart. A new service gitlab-http-router should now start.
5. In a new terminal tab, run gdk tail gitlab-http-router
6. Go to http://gdk.test:3000 (router). You should see traffic running via the router.
7. Go to http://gdk.test:3333 (workhorse). You should see no traffic running via the router.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.