[SPARK-17311][MLLIB] Standardize Python-Java MLlib API to accept optional long seeds in all cases

## What changes were proposed in this pull request?

Related to https://github.com/apache/spark/pull/14524 -- just the 'fix' rather than a behavior change.

- PythonMLlibAPI methods that take a seed now always take a `java.lang.Long` consistently, allowing the Python API to specify "no seed"
- .mllib's Word2VecModel seemed to be an odd man out in .mllib in that it picked its own random seed. Instead it defaults to None, meaning, letting the Scala implementation pick a seed
- BisectingKMeansModel arguably should not hard-code a seed for consistency with .mllib, I think. However I left it.

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #14826 from srowen/SPARK-16832.2.