Add option "use-existing-container" to run-db-test task

[tomuben]

Background

We plan to reorganize the CI flow of the SLC's. For that we need to pass the tar.gz of a container from previous steps to the next step: With that, we don't have the cached tar.gz available, and rebuilding the docker container and running export again would be too cost/time expansive.
The simplest and most elegant solution is to add a new option to the run-db-test task, which allows the usage of an "external" tar.gz for running the tests. It's then the responsibility of the CI to implement a mechanism to pass the .tar.gz from one step to the next.

Acceptance Criteria

1. Add a new option "--use-existing-container"
2. If this option is not None, the run-db-test must skip creation and export of the SLC, and use the given .tar.gz instead
3. There is no need to check if the given .tar.gz is compatible with the given flavor, but the documentation must make it clear, that it's the responsibility of the user of run-db-test task, that flavor and given .tar.gz match.