fix matrix report aggregation bug

We can't use ```endRun```, because endRun is not async. endRun isn't executed right after matrix build finishing, but it's executed according its queue. So, with using jclouds, often endRun is executed, when slave is shutdown already
You should have a class-level function for this, like:
```java
/**
 * Return path to put sub-run results for matrix build aggregation.
 */
protected FilePath getAggregationDir(Build build) {
    // TBD
}
```
You should call it from the ``startBuild`` and from the ``perform``'s block for the matrix run.