[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.

If the `seed` could be null, then we could use nanosecond for it in Scala.
```
if (seed == null) {
  ...
} else {
  ...
}
```