Add placeholder functions for "repmgr $command --help"

There are now too many options to sensibly fit into general --help
output; we'll add separate output for each repmgr command, e.g.
"repmgr node --help".

repmgr-action-cluster.c

@@ -1083,3 +1083,12 @@ cube_set_node_status(t_node_status_cube **cube, int n, int execute_node_id, int
 		}
 	}
 }
+
+
+void
+do_cluster_help(void)
+{
+	print_help_header();
+
+
+}