[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.

Oh, also, I left `seed` where it is in the python param list since both `seed` and `nonnegative` are optional.