[SPARK-1092] print warning information if user use SPARK_MEM to regulate executor memory usage

https://spark-project.atlassian.net/browse/SPARK-1092?jql=project%20%3D%20SPARK

print warning information if user set SPARK_MEM to regulate memory usage of executors

----
OUTDATED:

Currently, users will usually set SPARK_MEM to control the memory usage of driver programs, (in spark-class)
91 JAVA_OPTS="$OUR_JAVA_OPTS"
92 JAVA_OPTS="$JAVA_OPTS -Djava.library.path=$SPARK_LIBRARY_PATH"
93 JAVA_OPTS="$JAVA_OPTS -Xms$SPARK_MEM -Xmx$SPARK_MEM"
if they didn't set spark.executor.memory, the value in this environment variable will also affect the memory usage of executors, because the following lines in SparkContext
privatespark val executorMemory = conf.getOption("spark.executor.memory")
.orElse(Option(System.getenv("SPARK_MEM")))
.map(Utils.memoryStringToMb)
.getOrElse(512)
also
since SPARK_MEM has been (proposed to) deprecated in SPARK-929 (https://spark-project.atlassian.net/browse/SPARK-929) and the corresponding PR (https://github.com/apache/incubator-spark/pull/104)
we should remove this line

Author: CodingCat <zhunansjtu@gmail.com>

Closes #602 from CodingCat/clean_spark_mem and squashes the following commits:

302bb28 [CodingCat] print warning information if user use SPARK_MEM to regulate executor memory usage

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -170,6 +170,11 @@ class SparkContext(
     .map(Utils.memoryStringToMb)
     .getOrElse(512)
 
+  if (!conf.contains("spark.executor.memory") && sys.env.contains("SPARK_MEM")) {
+    logWarning("Using SPARK_MEM to set amount of memory to use per executor process is " +
+      "deprecated, instead use spark.executor.memory")
+  }
+
   // Environment variables to pass to our executors
   private[spark] val executorEnvs = HashMap[String, String]()
   // Note: SPARK_MEM is included for Mesos, but overwritten for standalone mode in ExecutorRunner