[SPARK-2682] Javadoc generated from Scala source code is not in javadoc's index

Add genjavadocSettings back to SparkBuild. It requires #1585 .

https://issues.apache.org/jira/browse/SPARK-2682

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1584 from yhuai/SPARK-2682 and squashes the following commits:

2e89461 [Yin Huai] Merge remote-tracking branch 'upstream/master' into SPARK-2682
54e3b66 [Yin Huai] Add genjavadocSettings back.