[SPARK-1696][MLLIB] use alpha in dense dspr

It doesn't affect existing code because only `alpha = 1.0` is used in the code.

Author: Xiangrui Meng <meng@databricks.com>

Closes #778 from mengxr/mllib-dspr-fix and squashes the following commits:

a37402e [Xiangrui Meng] use alpha in dense dspr

(cherry picked from commit e3d72a74ad007c2bf279d6a74cdaca948bdf0ddd)
Signed-off-by: Reynold Xin <rxin@apache.org>