Iceberg metadata table scanner should always be scheduled to the coordinator

On larger clusters the Iceberg metadata scanner can be scheduled to executors, for example during a join. The fragment in this case will fail a precondition check, because either the frontend_ object will not be present or the table. Setting exec_at_coord to true is not enough and these fragments should be scheduled to the coord_only_executor_group.

Additionally, setting NUM_NODES=1 should be a viable workaround.

Reproducible with the following local dev Impala cluster:
./bin/start-impala-cluster.py --cluster_size=3 --num_coordinators=1 --use_exclusive_coordinators
and query:
select count(b.parent_id) from functional_parquet.iceberg_query_metadata.history a
join functional_parquet.iceberg_query_metadata.history b on a.snapshot_id = b.snapshot_id;