[SPARK-10393] use ML pipeline in LDA example

jira: https://issues.apache.org/jira/browse/SPARK-10393

Since the logic of the text processing part has been moved to ML estimators/transformers, replace the related code in LDA Example with the ML pipeline.

Author: Yuhao Yang <hhbyyh@gmail.com>
Author: yuhaoyang <yuhao@zhanglipings-iMac.local>

Closes #8551 from hhbyyh/ldaExUpdate.