[SPARK-15362][ML] Make spark.ml KMeansModel load backwards compatible

## What changes were proposed in this pull request?
[SPARK-14646](https://issues.apache.org/jira/browse/SPARK-14646) makes ```KMeansModel``` store the cluster centers one per row. ```KMeansModel.load()``` method needs to be updated in order to load models saved with Spark 1.6.

## How was this patch tested?
Since ```save/load``` is ```Experimental``` for 1.6, I think offline test for backwards compatibility is enough.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #13149 from yanboliang/spark-15362.