[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.

mllib/src/main/scala/org/apache/spark/ml/clustering/KMeans.scala

@@ -185,6 +185,12 @@ object KMeansModel extends MLReadable[KMeansModel] {
   /** Helper class for storing model data */
   private case class Data(clusterIdx: Int, clusterCenter: Vector)
 
+  /**
+   * We store all cluster centers in a single row and use this class to store model data by
+   * Spark 1.6 and earlier. A model can be loaded from such older data for backward compatibility.
+   */
+  private case class OldData(clusterCenters: Array[OldVector])
+
   /** [[MLWriter]] instance for [[KMeansModel]] */
   private[KMeansModel] class KMeansModelWriter(instance: KMeansModel) extends MLWriter {
 
@@ -211,13 +217,19 @@ object KMeansModel extends MLReadable[KMeansModel] {
       import sqlContext.implicits._
 
       val metadata = DefaultParamsReader.loadMetadata(path, sc, className)
-
       val dataPath = new Path(path, "data").toString
-      val data: Dataset[Data] = sqlContext.read.parquet(dataPath).as[Data]
-      val clusterCenters = data.collect().sortBy(_.clusterIdx).map(_.clusterCenter)
-      val model = new KMeansModel(metadata.uid,
-        new MLlibKMeansModel(clusterCenters.map(OldVectors.fromML)))
 
+      val versionRegex = "([0-9]+)\\.(.+)".r
+      val versionRegex(major, _) = metadata.sparkVersion
+
+      val clusterCenters = if (major.toInt >= 2) {
+        val data: Dataset[Data] = sqlContext.read.parquet(dataPath).as[Data]
+        data.collect().sortBy(_.clusterIdx).map(_.clusterCenter).map(OldVectors.fromML)
+      } else {
+        // Loads KMeansModel stored with the old format used by Spark 1.6 and earlier.
+        sqlContext.read.parquet(dataPath).as[OldData].head().clusterCenters
+      }
+      val model = new KMeansModel(metadata.uid, new MLlibKMeansModel(clusterCenters))
       DefaultParamsReader.getAndSetParams(model, metadata)
       model
     }