[SPARK-10188] [PYSPARK] Pyspark CrossValidator with RMSE selects incorrect model

* Added isLargerBetter() method to Pyspark Evaluator to match the Scala version.
* JavaEvaluator delegates isLargerBetter() to underlying Scala object.
* Added check for isLargerBetter() in CrossValidator to determine whether to use argmin or argmax.
* Added test cases for where smaller is better (RMSE) and larger is better (R-Squared).

(This contribution is my original work and that I license the work to the project under Sparks' open source license)

Author: noelsmith <mail@noelsmith.com>

Closes #8399 from noel-smith/pyspark-rmse-xval-fix.