[MLLIB] [PYTHON] SPARK-4221: Expose nonnegative ALS in the python API

SPARK-1553 added alternating nonnegative least squares to MLLib, however it's not possible to access it via the python API.  This pull request resolves that.

Author: Michelangelo D'Agostino <mdagostino@civisanalytics.com>

Closes #3095 from mdagost/python_nmf and squashes the following commits:

a6743ad [Michelangelo D'Agostino] Use setters instead of static methods in PythonMLLibAPI.  Remove the new static methods I added.  Set seed in tests.  Change ratings to ratingsRDD in both train and trainImplicit for consistency.
7cffd39 [Michelangelo D'Agostino] Swapped nonnegative and seed in a few more places.
3fdc851 [Michelangelo D'Agostino] Moved seed to the end of the python parameter list.
bdcc154 [Michelangelo D'Agostino] Change seed type to java.lang.Long so that it can handle null.
cedf043 [Michelangelo D'Agostino] Added in ability to set the seed from python and made that play nice with the nonnegative changes.  Also made the python ALS tests more exact.
a72fdc9 [Michelangelo D'Agostino] Expose nonnegative ALS in the python API.

(cherry picked from commit 7e9d975676d56ace0e84c2200137e4cd4eba074a)
Signed-off-by: Xiangrui Meng <meng@databricks.com>