[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
This does more than fix the seed for unit tests, but for every call. Is it not a bit better to make the RNG injectable via a discreet package-private setter and let the tests inject a seeded RNG?