[FLINK-1139] Fix for HadoopOF with DOP > 1 

This PR allows to execute HadoopOFs with DOP > 1.
It adds a hook to JM that is executed after the last OutputFormat finished. 
Hadoop FileOutputFormats need this hook to clean-up temporary files.
This method is the last thing that runs from an operator. I would make all variables stack variables, not fields of the class.