Is there a way to choose a version of cassandra 3.6+ builtin into the config system or we have to install cassandra manually if we want this?
Are you running on 1.0 or 2.0?
2.0 makes this very simple. In this example: https://circleci.com/docs/2.0/language-ruby/#sample-configuration you can replace PostgreSQL with whatever version of Cassandra you want to use and it will be available on localhost