Use the server certificate in the NodeJS example

[sat0yu]

Is your feature request related to a problem? Please describe.
The document of using MQTT protocol in Cloud IoT Core mentions the server certificate.
The NodeJS example, however, seems to be ignoring that part in the code.

Describe the solution you'd like
Since all other examples, C++, JAVA, and Python support TLS connection with the server certificate, the one for NodeJS might be better off doing so as well.

• ref. C++ https://github.com/GoogleCloudPlatform/cpp-samples/blob/master/iot/mqtt-ciotc/mqtt_ciotc.c#L68

Describe alternatives you've considered
none

Additional context
none

[gcseh]

Thank you for raising this request to us. It has been implemented with #438 .
Therefore I will close #423 .