Avoid using test.repl->N in MariaDBMonitor tests

The value may not match the nodes used by the test.

system-test/mariadbmonitor/mysqlmon_failover_rolling_restart_slaves.cpp

@@ -158,8 +158,7 @@ void run(TestConnections& test)
 {
     test.maxscales->wait_for_monitor();
 
-    int N = test.repl->N;
-    cout << "Nodes: " << N << endl;
+    const int N = 4;
 
     check_server_status(test, N);