[SPARK-9447] [ML] [PYTHON] Added HasRawPredictionCol, HasProbabilityCol to RandomForestClassifier

Added HasRawPredictionCol, HasProbabilityCol to RandomForestClassifier, plus doc tests for those columns.

CC: holdenk yanboliang

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

Closes #7903 from jkbradley/rf-prob-python and squashes the following commits:

c62a83f [Joseph K. Bradley] made unit test more robust
14eeba2 [Joseph K. Bradley] added HasRawPredictionCol, HasProbabilityCol to RandomForestClassifier in PySpark