[SPARK-2495][MLLIB] make KMeans constructor public

to re-construct k-means models freeman-lab

Author: Xiangrui Meng <meng@databricks.com>

Closes #2112 from mengxr/public-constructors and squashes the following commits:

18d53a9 [Xiangrui Meng] make KMeans constructor public

mllib/src/main/scala/org/apache/spark/mllib/clustering/KMeansModel.scala

@@ -25,7 +25,7 @@ import org.apache.spark.mllib.linalg.Vector
 /**
  * A clustering model for K-means. Each point belongs to the cluster with the closest center.
  */
-class KMeansModel private[mllib] (val clusterCenters: Array[Vector]) extends Serializable {
+class KMeansModel (val clusterCenters: Array[Vector]) extends Serializable {
 
   /** Total number of clusters. */
   def k: Int = clusterCenters.length