[BACKLOG-350] Prevents the reducer from initializing the Kettle environm...

...ent for each invocation. There are tests for this in pentaho-big-data-plugin. I'll send a second pull request for pentaho-big-data-plugin as well, since the test was not closing the buffers and results in errors. I've tested the change on CDH50 and PDI 5.1.0.0-752. Both the single-threaded and the multi-threaded reducers work perfectly.

The tests for this request are in this other pull request:

https://github.com/pentaho/big-data-plugin/pull/246