Fix scalastyle warnings in yarn alpha

Author: witgo <witgo@qq.com>

Closes #884 from witgo/scalastyle and squashes the following commits:

4b08ae4 [witgo] Fix scalastyle warnings in yarn alpha

dev/scalastyle

@@ -19,7 +19,8 @@
 
 echo -e "q\n" | SPARK_HIVE=true sbt/sbt scalastyle > scalastyle.txt
 # Check style with YARN alpha built too
-echo -e "q\n" | SPARK_YARN=true sbt/sbt yarn/scalastyle >> scalastyle.txt
+echo -e "q\n" | SPARK_HADOOP_VERSION=0.23.9 SPARK_YARN=true sbt/sbt yarn-alpha/scalastyle \
+  >> scalastyle.txt
 # Check style with YARN built too
 echo -e "q\n" | SPARK_HADOOP_VERSION=2.2.0 SPARK_YARN=true sbt/sbt yarn/scalastyle \
   >> scalastyle.txt