[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.
BLOCKER: This here introduces a critical bug. This function is not meant to abort with an exception. It should be
```
try {
    getJobVertex().finalizeOnMaster(getGraph().getUserClassLoader());
}
catch (Throwable t) {
    getGraph().fail(t);
```