[SPARK-4970] Do not read spark.executor.memory from spark-defaults.conf in SparkSubmitSuite

The test 'includes jars passed in through --jars’ in SparkSubmitSuite fails
when spark.executor.memory is set at over 512MiB in conf/spark-default.conf.
An exception is thrown as follows:
Exception in thread "main" org.apache.spark.SparkException: Asked to launch cluster with 512 MB RAM / worker but requested 1024 MB/worker
at org.apache.spark.SparkContext$.org$apache$spark$SparkContext$$createTaskScheduler(SparkContext.scala:1889)
at org.apache.spark.SparkContext.(SparkContext.scala:322)
at org.apache.spark.deploy.JarCreationTest$.main(SparkSubmitSuite.scala:458)
at org.apache.spark.deploy.JarCreationTest.main(SparkSubmitSuite.scala)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.apache.spark.deploy.SparkSubmit$.launch(SparkSubmit.scala:367)
at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:75)
at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)
spark.executor.memory can be loaded from spark-default.conf correctly. However, '--executor-memory' takes no effect in local mode.
Not sure about why it is.