[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

(cherry picked from commit 220f413686ae922bd11776576bf37610cce92c23)
Signed-off-by: Xiangrui Meng <meng@databricks.com>