Protect tier2 images with locking mechanism

The tier2 image garbage collector has been enabled in production and now a tier2 image might be removed by the gcvms script. There is a race condition that might cause a provisioned template or image to be removed by the script. Therefore there has to be a lock mechanism to protect a tier2 image from being garbage collected by accident.

The vm template names need a locking mechanism after which patch https://codereview.qt-project.org/c/qtqa/tqtc-coin-ci/+/256747 can be applied to allow removing the cache.