Re-connect mqtt

Re-establish mqtt connection, the premise is that this mqtt connection must have been established at last time by correct Iot_id, Iot_pwd.

Request:

"|4|1|5|\r"

Returns:

"|4|1|1|1|\r" mqtt connection status: successfully connected

"|4|1|1|2|reason|\r" mqtt connection status: failed to connect, the reason string indicates the reason why it failed.

Arduino Sample:

softSerial.print("|4|1|5|\r");