Improve scripts/setup-ssh-tunnel.sh implementation

Based on https://github.com/guardian/workflow-frontend/pull/537, we had some issues running the script after some changes in AWS. It would be nice for the script to be more resilient to LB changes. Furthermore it seems that this script could benefit from some tidying up https://github.com/guardian/workflow-frontend/pull/537#pullrequestreview-3448837878. 