[SPARK-9440] [MLLIB] Add hyperparameters to LocalLDAModel save/load

jkbradley MechCoder

Resolves blocking issue for SPARK-6793. Please review after #7705 is merged.

Author: Feynman Liang <fliang@databricks.com>

Closes #7757 from feynmanliang/SPARK-9940-localSaveLoad and squashes the following commits:

d0d8cf4 [Feynman Liang] Fix thisClassName
0f30109 [Feynman Liang] Fix tests after changing LDAModel public API
dc61981 [Feynman Liang] Add hyperparams to LocalLDAModel save/load