[SPARK-8953] SPARK_EXECUTOR_CORES is not read in SparkSubmit

The configuration ```SPARK_EXECUTOR_CORES``` won't put into ```SparkConf```, so it has no effect to the dynamic executor allocation.

Author: xutingjun <xutingjun@huawei.com>

Closes #7322 from XuTingjun/SPARK_EXECUTOR_CORES and squashes the following commits:

2cafa89 [xutingjun] make SPARK_EXECUTOR_CORES has effect to dynamicAllocation

core/src/main/scala/org/apache/spark/deploy/SparkSubmitArguments.scala

@@ -162,6 +162,7 @@ private[deploy] class SparkSubmitArguments(args: Seq[String], env: Map[String, S
       .orNull
     executorCores = Option(executorCores)
       .orElse(sparkProperties.get("spark.executor.cores"))
+      .orElse(env.get("SPARK_EXECUTOR_CORES"))
       .orNull
     totalExecutorCores = Option(totalExecutorCores)
       .orElse(sparkProperties.get("spark.cores.max"))