Fixes bug with PySpark cvModel not having UID
Also made small PySpark fixes: Evaluator should inherit from Params.  MockModel should inherit from Model.

CC: mengxr

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

Closes #5968 from jkbradley/pyspark-cv-uid and squashes the following commits:

57f13cd [Joseph K. Bradley] Made CrossValidatorModel call parent init in PySpark