HPCC-11443 Allow Roxie to specify remote cluster priorities

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5952 from richardkchapman/roxie-preferred-clusters

HPCC-11443 Allow Roxie to specify remote cluster priorities

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>