Consider creating a pool of review apps to run multiple test jobs against

When we start using review apps to test orchestrated scenarios, it might be useful to be able to create a pool of review apps instead of always running one review app per job (which is what we do now in package-and-qa -- one GitLab docker container per job).

The way it might work is that when gitlab-qa is going to start a scenario, it could:

1. Check if a review app with a suitable configuration is available
   • if not, start one (either by modifying the config of an existing idle review app, or starting a new one)
   • or if one is available, use it

Then when the scenario is complete, if there are no pending jobs the review app is shut down, otherwise it's marked as idle and a pending jobs can use it.