Wait for monitor in bug547

The test expects the monitor to respond within 5 seconds. This is not
guaranteed to happen so waiting for a monitor interval instead of a
hard-coded duration provides for a more stable test.

maxscale-system-test/bug547.cpp

@@ -35,7 +35,7 @@ int main(int argc, char *argv[])
     }
 
     Test->set_timeout(30);
-    sleep(5);
+    Test->maxscales->wait_for_monitor();
 
     Test->set_timeout(30);
     Test->tprintf("Connecting to all MaxScale services, expecting error\n");