[MLLIB] set RDD names in ALS

This is very useful when debugging & fine tuning jobs with large data sets.

Author: Neville Li <neville@spotify.com>

Closes #966 from nevillelyh/master and squashes the following commits:

6747764 [Neville Li] [MLLIB] use string interpolation for RDD names
3b15d34 [Neville Li] [MLLIB] set RDD names in ALS