Update versions and minor cleanups

[Gerrit91]

Adapting to freshly deployed CI runners, which look a bit different now.

• Only delete workspace directory if already exists (robustness against fresh CI runners)
• Use SSH keys from this project folder for machine creation (then users don't need to create an own key before, also fresh CI runners don't need a key)
• Bump to metal-stack v0.9.1
• Remove unnecessary libvirt cleanup (remainings from mini-lab version without containerlab)
• Add -T flag for CI (required in docker-compose v2)