Fix swagger-ui for instances with relative URL

Fixes #388351 (closed)

What does this MR do and why?

OpenAPI viewer doesn't consider instances with relative url, so this MR fixes that issue.

Screenshots or screen recordings

How to set up and validate locally

1. Configure relative url for your GDK instance (gdk.yml)

hostname: gdk.test
relative_url_root: /rel

2. gdk restart
3. Visit gdk.test:3000/rel and configure .yml

MR acceptance checklist

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