[SPARK-4465] runAsSparkUser doesn't affect TaskRunner in Mesos environme...

...nt at all.

- fixed a scope of runAsSparkUser from MesosExecutorDriver.run to MesosExecutorBackend.launchTask
- See the Jira Issue for more details.

Author: Jongyoul Lee <jongyoul@gmail.com>

Closes #3741 from jongyoul/SPARK-4465 and squashes the following commits:

46ad71e [Jongyoul Lee] [SPARK-4465] runAsSparkUser doesn't affect TaskRunner in Mesos environment at all. - Removed unused import
3d6631f [Jongyoul Lee] [SPARK-4465] runAsSparkUser doesn't affect TaskRunner in Mesos environment at all. - Removed comments and adjusted indentations
2343f13 [Jongyoul Lee] [SPARK-4465] runAsSparkUser doesn't affect TaskRunner in Mesos environment at all. - fixed a scope of runAsSparkUser from MesosExecutorDriver.run to MesosExecutorBackend.launchTask