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