cluster.js calls an undefined function to iterate over the config server nodes

SERVER-58683 changed the way cluster.js iterates through the nodes of the config server replica set to perform several operations (such as executeOnConfigNodes()) by relying on the st.forEachConfigServer function.
 
Nevertheless, such function is undefined in Cluster.st, which is an object of type FSMShardingTest (and not ShardingTest).

(The consequences of this may go unperceived when executeOnConfigNodes() gets invoked in the setup() of an FSM workload: an uncaught exception will appear in the test log and no operation will be applied to the config server nodes, but the workload itself will be executed anyway, and possibly pass the test assertions).