[SPARK-10332] [CORE] Fix yarn spark executor validation

From Jira:
Running spark-submit with yarn with number-executors equal to 0 when not using dynamic allocation should error out.
In spark 1.5.0 it continues and ends up hanging.
yarn.ClientArguments still has the check so something else must have changed.
spark-submit --master yarn --deploy-mode cluster --class org.apache.spark.examples.SparkPi --num-executors 0 ....
spark 1.4.1 errors with:
java.lang.IllegalArgumentException:
Number of executors was 0, but must be at least 1
(or 0 if dynamic executor allocation is enabled).

Author: Holden Karau <holden@pigscanfly.ca>

Closes #8580 from holdenk/SPARK-10332-spark-submit-to-yarn-executors-0-message.

yarn/src/main/scala/org/apache/spark/deploy/yarn/ClientArguments.scala

@@ -96,6 +96,9 @@ private[spark] class ClientArguments(args: Array[String], sparkConf: SparkConf)
       }
 
       numExecutors = initialNumExecutors
+    } else {
+      val numExecutorsConf = "spark.executor.instances"
+      numExecutors = sparkConf.getInt(numExecutorsConf, numExecutors)
     }
     principal = Option(principal)
       .orElse(sparkConf.getOption("spark.yarn.principal"))