pytest_plugin: Improve cassandra fixtures to speedup and cleanup tests

Modify swh_storage_cassandra_cluster to ensure a single instance of cassandra is spawned when running tests in parallel with pytest-xdist.

Explicitly shutdown clusters create by CqlRunner instances to avoid the display of traceback related to Python threads when cassandra related fixtures are tear downed.

These changes significantly reduce the time to execute a tox session (more than a x2 speedup).