fix(artifacts_test): ignore absence of perftune.yaml
fix(artifacts_test): ignore absence of perftune.yaml

There is no perftune.yaml on Docker image, so, return None (unset) for
write_back_cache setting in that case.

scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests