[SPARK-8387] [FOLLOWUP ] [WEBUI] Update driver log URL to show only 4096 bytes

This is to follow up #6834 , update the driver log URL as well for consistency.

Author: Carson Wang <carson.wang@intel.com>

Closes #6878 from carsonwang/logUrl and squashes the following commits:

13be948 [Carson Wang] update log URL in YarnClusterSuite
a0004f4 [Carson Wang] Update driver log URL to show only 4096 bytes

Author: carsonwang

yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnClusterSchedulerBackend.scala

@@ -115,8 +115,9 @@ private[spark] class YarnClusterSchedulerBackend(
           val httpScheme = if (yarnHttpPolicy == "HTTPS_ONLY") "https://" else "http://"
           val baseUrl = s"$httpScheme$httpAddress/node/containerlogs/$containerId/$user"
           logDebug(s"Base URL for logs: $baseUrl")
-          driverLogs = Some(
-            Map("stderr" -> s"$baseUrl/stderr?start=0", "stdout" -> s"$baseUrl/stdout?start=0"))
+          driverLogs = Some(Map(
+            "stderr" -> s"$baseUrl/stderr?start=-4096",
+            "stdout" -> s"$baseUrl/stdout?start=-4096"))
         }
       }
     } catch {

yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnClusterSuite.scala

@@ -376,7 +376,7 @@ private object YarnClusterDriver extends Logging with Matchers {
       new URL(urlStr)
       val containerId = YarnSparkHadoopUtil.get.getContainerId
       val user = Utils.getCurrentUserName()
-      assert(urlStr.endsWith(s"/node/containerlogs/$containerId/$user/stderr?start=0"))
+      assert(urlStr.endsWith(s"/node/containerlogs/$containerId/$user/stderr?start=-4096"))
     }
   }