[SPARK-2864][MLLIB] fix random seed in word2vec; move model to local

It also moves the model to local in order to map `RDD[String]` to `RDD[Vector]`.

Ishiihara

Author: Xiangrui Meng <meng@databricks.com>

Closes #1790 from mengxr/word2vec-fix and squashes the following commits:

a87146c [Xiangrui Meng] add setters and make a default constructor
e5c923b [Xiangrui Meng] fix random seed in word2vec; move model to local