CassandraStressThread: save c-s log inside subdirectory of each loader
CassandraStressThread: save c-s log inside subdirectory of each loader

Currently the c-s log of all loaders will be saved in base log directory of
loader cluster.

$ find latest/rolling-upgrade-upgrade--centos-loader-set-99b33370/
latest/rolling-upgrade-upgrade--centos-loader-set-99b33370/
latest/rolling-upgrade-upgrade--centos-loader-set-99b33370/rolling-upgrade-upgrade--centos-loader-node-99b33370-0-1
latest/rolling-upgrade-upgrade--centos-loader-set-99b33370/cassandra-stress-l0-c0-k1-157f495c-fd49-4960-81ab-bd6d106c7980.log

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

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