feature(cluster.py): support new directory structure of unified package
feature(cluster.py): support new directory structure of unified package

On https://github.com/scylladb/scylladb/pull/8867, we changed directory
structure of unified package, and increment it's
relocatable_package_version to 3.0.

We need to support it on SCT to run offline tests.

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