COORDINATOR() functionality is available from CDH 6.1 which returns the name of the host which is running the impalad daemon that is acting as the coordinator for the current query.
For the version CDH 5.x, if you want to see the coordinator or the host there is a simple trick.
Run any select statement. Before it returns any result, it shows where the query is submitted.
If you look at the above screenshot, the date and time of submission of the query, along with the coordinator to which the query is sent, is stated.
In this case, the coordinator is – http://quickstart.cloudera:25000.