Update keepalive vale

We need a shorter keepalive because the server setting is 60 second, we have seen 30 is good enough for most cases and makes a more reliable connection.

Author: mastrolinux

src/ArduinoIoTCloud.h

@@ -26,7 +26,7 @@ class ArduinoIoTCloudClass {
   static const int MQTT_BUFFER_SIZE = 256;
   static const int MAX_RETRIES = 5;
   static const int RECONNECTION_TIMEOUT = 2000;
-  const mqttConnectionOptions mqttOpt = {120, false, 1000};
+  const mqttConnectionOptions mqttOpt = {30, false, 1000};
 
   int connect();
   bool disconnect();