In the default Spark distribution, there are currently two separate
log4j config files, with different default values for the root logger,
so that when running the shell you have a different default log level.
This makes the shell more usable, since the logs don't overwhelm the
output.

But if you install a custom log4j.properties, you lose that, because
then it's going to be used no matter whether you're running a regular
app or the shell.

With this change, the overriding of the log level is done differently;
the log level repl's main class (org.apache.spark.repl.Main) is used
to define the root logger's level when running the shell, defaulting
to WARN if it's not set explicitly.

On a somewhat related change, the shell output about the "sc" variable
was changed a bit to contain a little more useful information about
the application, since when the root logger's log level is WARN, that
information is never shown to the user.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #9816 from vanzin/shell-logging.