[SPARK-8484] [ML] Added TrainValidationSplit for hyper-parameter tuning.

- [X] Added TrainValidationSplit for hyper-parameter tuning. It randomly splits the input dataset into train and validation and use evaluation metric on the validation set to select the best model. It should be similar to CrossValidator, but simpler and less expensive.
- [X] Simplified replacement of https://github.com/apache/spark/pull/6996

Author: martinzapletal <zapletal-martin@email.cz>

Closes #7337 from zapletal-martin/SPARK-8484-TrainValidationSplit and squashes the following commits:

cafc949 [martinzapletal] Review comments https://github.com/apache/spark/pull/7337.
511b398 [martinzapletal] Merge remote-tracking branch 'upstream/master' into SPARK-8484-TrainValidationSplit
f4fc9c4 [martinzapletal] SPARK-8484 Resolved feedback to https://github.com/apache/spark/pull/7337
00c4f5a [martinzapletal] SPARK-8484. Styling.
d699506 [martinzapletal] SPARK-8484. Styling.
93ed2ee [martinzapletal] Styling.
3bc1853 [martinzapletal] SPARK-8484. Styling.
2aa6f43 [martinzapletal] SPARK-8484. Added TrainValidationSplit for hyper-parameter tuning. It randomly splits the input dataset into train and validation and use evaluation metric on the validation set to select the best model.
21662eb [martinzapletal] SPARK-8484. Added TrainValidationSplit for hyper-parameter tuning. It randomly splits the input dataset into train and validation and use evaluation metric on the validation set to select the best model.