[SPARK-5396] Syntax error in spark scripts on windows.

Modified syntax error in spark-submit2.cmd. Command prompt doesn't have "defined" operator.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #4428 from tsudukim/feature/SPARK-5396 and squashes the following commits:

ec18465 [Masayoshi TSUZUKI] [SPARK-5396] Syntax error in spark scripts on windows.

bin/spark-submit2.cmd

@@ -25,7 +25,7 @@ set ORIG_ARGS=%*
 rem Reset the values of all variables used
 set SPARK_SUBMIT_DEPLOY_MODE=client
 
-if not defined %SPARK_CONF_DIR% (
+if [%SPARK_CONF_DIR%] == [] (
   set SPARK_CONF_DIR=%SPARK_HOME%\conf
 )
 set SPARK_SUBMIT_PROPERTIES_FILE=%SPARK_CONF_DIR%\spark-defaults.conf