JIRA: https://issues.apache.org/jira/browse/SPARK-10524

Currently we use the hard prediction (`ImpurityCalculator.predict`) to order categories' bins. But we should use the soft prediction.

Author: Liang-Chi Hsieh <viirya@gmail.com>
Author: Liang-Chi Hsieh <viirya@appier.com>
Author: Joseph K. Bradley <joseph@databricks.com>

Closes #8734 from viirya/dt-soft-centroids.