[SPARK-19866][ML][PYSPARK] Add local version of Word2Vec findSynonyms for spark.ml: Python API

https://issues.apache.org/jira/browse/SPARK-19866

## What changes were proposed in this pull request?

Add Python API for findSynonymsArray matching Scala API.

## How was this patch tested?

Manual test
`./python/run-tests --python-executables=python2.7 --modules=pyspark-ml`

Author: Xin Ren <iamshrek@126.com>
Author: Xin Ren <renxin.ubc@gmail.com>
Author: Xin Ren <keypointt@users.noreply.github.com>

Closes #17451 from keypointt/SPARK-19866.