[SPARK-7329] [MLLIB] simplify ParamGridBuilder impl

as suggested by justinuang on #5601.

Author: Xiangrui Meng <meng@databricks.com>

Closes #5873 from mengxr/SPARK-7329 and squashes the following commits:

d08f9cf [Xiangrui Meng] simplify tests
b7a7b9b [Xiangrui Meng] simplify grid build