[SPARK-14845][SPARK_SUBMIT][YARN] spark.files in properties file is n…

## What changes were proposed in this pull request?

initialize SparkSubmitArgument#files first from spark-submit arguments then from properties file, so that sys property spark.yarn.dist.files will be set correctly.
```
OptionAssigner(args.files, YARN, ALL_DEPLOY_MODES, sysProp = "spark.yarn.dist.files"),
```
## How was this patch tested?

manul test. file defined in properties file is also distributed to driver in yarn-cluster mode.

Author: Jeff Zhang <zjffdu@apache.org>

Closes #12656 from zjffdu/SPARK-14845.

(cherry picked from commit 0a30269)
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>

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

@@ -173,6 +173,7 @@ private[deploy] class SparkSubmitArguments(args: Seq[String], env: Map[String, S
       .orNull
     name = Option(name).orElse(sparkProperties.get("spark.app.name")).orNull
     jars = Option(jars).orElse(sparkProperties.get("spark.jars")).orNull
+    files = Option(files).orElse(sparkProperties.get("spark.files")).orNull
     ivyRepoPath = sparkProperties.get("spark.jars.ivy").orNull
     packages = Option(packages).orElse(sparkProperties.get("spark.jars.packages")).orNull
     packagesExclusions = Option(packagesExclusions)