[SPARK-9645] [YARN] [CORE] Allow shuffle service to read shuffle files.

Spark should not mess with the permissions of directories created
by the cluster manager. Here, by setting the block manager dir
permissions to 700, the shuffle service (running as the YARN user)
wouldn't be able to serve shuffle files created by applications.

Also, the code to protect the local app dir was missing in standalone's
Worker; that has been now added. Since all processes run as the same
user in standalone, `chmod 700` should not cause problems.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7966 from vanzin/SPARK-9645 and squashes the following commits:

6e07b31 [Marcelo Vanzin] Protect the app dir in standalone mode.
384ba6a [Marcelo Vanzin] [SPARK-9645] [yarn] [core] Allow shuffle service to read shuffle files.