[experiments] Multiple database versions are shown at the configurator

We have just inserted a new database version for AT&T, called atnt/2. The insertion works fine and the database listing flawlessly.

Meanwhile, when configuring experiments, both database versions are still display whereas only the latest one should be.

Ideally, we need this fixed for the tutorial. @philip.abbet: Do you think you can have a look at this ASAP? (thanks!)