fill_db_data: use independent keyspace name
fill_db_data: use independent keyspace name

fill_db_data test will drop test keyspaces in clean stage, currently it shares
keyspace1 with cs workloads, it might effect running workloads.

v2: use session.set_keyspace interface, remove repeated setting of keyspace (Bentsi)

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

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