MXS-3410 Rename Mariadb_nodes::flush_hosts() to prepare_for_test()

The function does more than simply executes "FLUSH HOSTS". Also
change return type to bool as that is what it has returned.

system-test/auth_change_user_loop.cpp

@@ -109,7 +109,8 @@ int main(int argc, char* argv[])
         pthread_join(parall_traffic1[j], NULL);
     }
     Test->tprintf("All threads are finished");
-    Test->repl->flush_hosts();
+    // TODO: Just a simple flush_hosts() may be sufficient.
+    Test->repl->prepare_for_test();
 
     Test->tprintf("Change user to '%s' in order to be able to DROP user", Test->maxscales->user_name);
     Test->set_timeout(30);