[MLlib] Correctly set vectorSize and alpha

mengxr
Correctly set vectorSize and alpha in Word2Vec training.

Author: Liquan Pei <liquanpei@gmail.com>

Closes #1900 from Ishiihara/Word2Vec-bugfix and squashes the following commits:

85f64f2 [Liquan Pei] correctly set vectorSize and alpha

(cherry picked from commit f0060b75ff67ab60babf54149a6860edc53cb6e9)
Signed-off-by: Xiangrui Meng <meng@databricks.com>