[SPARK-20354][CORE][REST-API] When I request access to the 'http:...

[SPARK-20354][CORE][REST-API] When I request access to the 'http: //ip:port/api/v1/applications' link, return 'sparkUser' is empty in REST API.

## What changes were proposed in this pull request?

When I request access to the 'http: //ip:port/api/v1/applications' link, get the json. I need the 'sparkUser' field specific value, because my Spark big data management platform needs to filter through this field which user submits the application to facilitate my administration and query, but the current return of the json string is empty, causing me this Function can not be achieved, that is, I do not know who the specific application is submitted by this REST Api.

**current return json:**
[ {
  "id" : "app-20170417152053-0000",
  "name" : "KafkaWordCount",
  "attempts" : [ {
    "startTime" : "2017-04-17T07:20:51.395GMT",
    "endTime" : "1969-12-31T23:59:59.999GMT",
    "lastUpdated" : "2017-04-17T07:20:51.395GMT",
    "duration" : 0,
    **"sparkUser" : "",**
    "completed" : false,
    "endTimeEpoch" : -1,
    "startTimeEpoch" : 1492413651395,
    "lastUpdatedEpoch" : 1492413651395
  } ]
} ]

**When I fix this question, return json:**
[ {
  "id" : "app-20170417154201-0000",
  "name" : "KafkaWordCount",
  "attempts" : [ {
    "startTime" : "2017-04-17T07:41:57.335GMT",
    "endTime" : "1969-12-31T23:59:59.999GMT",
    "lastUpdated" : "2017-04-17T07:41:57.335GMT",
    "duration" : 0,
    **"sparkUser" : "mr",**
    "completed" : false,
    "startTimeEpoch" : 1492414917335,
    "endTimeEpoch" : -1,
    "lastUpdatedEpoch" : 1492414917335
  } ]
} ]

## How was this patch tested?

manual tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: 郭小龙 10207633 <guo.xiaolong1@zte.com.cn>
Author: guoxiaolong <guo.xiaolong1@zte.com.cn>
Author: guoxiaolongzte <guo.xiaolong1@zte.com.cn>

Closes #17656 from guoxiaolongzte/SPARK-20354.

core/src/main/scala/org/apache/spark/ui/SparkUI.scala

@@ -117,7 +117,7 @@ private[spark] class SparkUI private (
         endTime = new Date(-1),
         duration = 0,
         lastUpdated = new Date(startTime),
-        sparkUser = "",
+        sparkUser = getSparkUser,
         completed = false
       ))
     ))