FLINK-1276

Fixes FLINK-1276
We actually want to make the execution mode configurable with a setter method, so this would actually be god to stay.

We are thinking about how to do the configuration:
  - A set of setters
  - A configuration/parameter object

I am leaning towards the second option, because we will have many more things to configure (closure cleaning, custom serializers, file system modes, mutable object modes)