Refactor of Websocket wrapper to use promise.

[dlabordus]

@juancho0202 as already told a nicer version for using websocket.

The websocket function now returns a Promise like most rest calls are also doing after some response handling.
If something goes wrong a reject is done with the reason body as used in other places with rest calls.
This way the handling if websocket or rest should be used could be hidden in the service layer.
Calling code doesn't know anymore.

Remark: I created a local container image and used the Docker Compose file and robot test script to validate the change.

[juancho0202]

I really like how you got rid of the _UsingWebsockets and _UsingRest in so many components. Code is defo more straight-forward. My only comment I consider non-blocking, therefore I approved already.