Use env var for Grafana app URL

[mderynck]

What this PR does

Bypasses cfg.AppURL() since it is currently returning localhost although GF_APP_URL is set in the environment.

Which issue(s) this PR closes

Closes [issue link here]

Checklist

• [ ] Unit, integration, and e2e (if applicable) tests updated
• [ ] Documentation added (or pr:no public docs PR label added if not required)
• [ ] Added the relevant release notes label (see labels prefixed w/ release:). These labels dictate how your PR will show up in the autogenerated release notes.