Allow disabling of new sign ups for tunnel admin user

What does this MR do?

In order to re-enable safely the QA Tunnel, we are ensuring new flows are in place before the specs run on the GitLab instance in the CI pipeline:

• disable signups ( 👈 you are here)
• change the root password to a secure tunnel password

Context: https://gitlab.com/gitlab-com/gl-security/security-operations/sirt/operations/-/issues/1060#note_426100103 (the first two points)

The creation of a new parallel scenario for Tunnel (instead of Kubernetes) will help migrate and test iteratively the new approach:

This is a new scenario that is part of refactoring the existing Kubernetes scenario. The tests that need an ssh tunnel now are not necessarily related to Kubernetes (and the connectivity to GKE), and the name is overarching to testing the Container Registry, Auto DevOps, Monitoring Clusters with Prometheus, and other features that depend on Internet routing. It is important to keep the existing Kubernetes scenario while testing and migrating.