[SPARK-9268] [ML] Removed varargs annotation from Params.setDefault taking multiple params

Removed varargs annotation from Params.setDefault taking multiple params.

Though varargs is technically correct, it often requires that developers do clean assembly, rather than (not clean) assembly, which is a nuisance during development.

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #7604 from jkbradley/params-setdefault-varargs and squashes the following commits:

6016dc6 [Joseph K. Bradley] removed varargs annotation from Params.setDefault taking multiple params