Allow users to pass custom user id on self-hosted troubleshooting rake task

What does this MR do and why?

gitlab:duo:verify_self_hosted_setup to accept an optional parameter for username, so administrators can run the troubleshoot with a different user than root since root might not always have an assigned seat

AI_GATEWAY_URL="http://gdk.test:5052" bundle exec rake "gitlab:duo:verify_self_hosted_setup[username]"

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #506806 (closed)