- SecurityManager complains when auth is enabled but no secret is defined;
  SparkSubmit doesn't use the auth functionality of the SecurityManager,
  so use a dummy secret to work around the exception.

- Only reset the log4j configuration when Spark was the one initializing
  it, otherwise user-defined log configuration may be lost.

Tested with the log config file posted to the bug, on a secured YARN cluster.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #19089 from vanzin/SPARK-21728.