Update client to use new IOT API from Viessmann.
Removed session_start() from code so that session is managed by the user of the api.
credentials.properties is now defined as key=value
Note: clientId is now mandatory and can be acquired on developper's portal https://developer.viessmann.com/
the clientId is the key delivered by a user to a third-party application. This key allows the third-party application(this code here) to access IOT API.
Once you got the key you have to set it into the credentials.properties into clientId field