[SPARK-4913] Fix incorrect event log path

SPARK-2261 uses a single file to log events for an app. `eventLogDir` in `ApplicationDescription` is replaced with `eventLogFile`. However, `ApplicationDescription` in `SparkDeploySchedulerBackend` is initialized with `SparkContext`'s `eventLogDir`. It is just the log directory, not the actual log file path. `Master.rebuildSparkUI` can not correctly rebuild a new SparkUI for the app.

Because the `ApplicationDescription` is remotely registered with `Master` and the app's id is then generated in `Master`, we can not get the app id in advance before registration. So the received description needs to be modified with correct `eventLogFile` value.