[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.

We could use java.lang.Long for seed, then seed can be null (means not specified), then we do not need another stub.

Also, putting `seed` in the end may be better (`seed` is optional).