HPCC-10710 Implement getClusterName() in Roxie

Part of the work required for hthor retirement

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5367 from richardkchapman/roxie-getclustername

HPCC-10710 Implement getClusterName() in Roxie

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>