The default values of data_file_directories and commitlog_directory were
commented by commit e0f40ed16a. It causes scylla_util.py:get_scylla_dirs() to
fail in checking the values.
This patch changed get_scylla_dirs() to return default data/commitlog
directories if they aren't set.
Fixes #5358
Reviewed-by: Pavel Emelyanov <xemul@scylladb.com>
Signed-off-by: Amos Kong <amos@scylladb.com>