Override default testcontainer image

[Alexander-Krause-Glau]

Due to #192, we should be able to override the default Dockerhub cassandra:latest image via for example quarkus.cassandra.test.container.image=cassandra:3.11.14 in the application.properties file.

Is this broken? Because it does not work on my side. Reproducer:

1. Clone https://github.com/ExplorViz/landscape-service/tree/cassandra-image-int-tests
2. Checkout cassandra-image-int-tests branch
3. docker imagesto verify that no cassandra image is locally available
4. Run ./gradlew integrationTest
5. Regardless of the test outcome, cassandra:latest was downloaded and not cassandra:3.11.14.

┆Issue is synchronized with this Jira Task by Unito