[FLINK-1008] Fix createProgramPlan() throws exception

Problem was, that ExecutionEnvironment#getExecutionPlan clears the data
sinks, i.e. a following ExecutionEnvironment#execute will throw an error
because there are no data sinks.

This introduces a new flag for ExecutionEnvironment#createProgramPlan to
indicate, that the the sinks shall not be cleared.
This does not break any existing code.