Reduce the amount of duplicate logging Akka does to stdout.

Given we have Akka logging go through SLF4j to log4j, we don't need
all the extra noise of Akka's stdout logger that is supposedly only
used during Akka init time but seems to continue logging lots of
noisy network events that we either don't care about or are in the
log4j logs anyway.

See:

http://doc.akka.io/docs/akka/2.0/general/configuration.html

    # Log level for the very basic logger activated during AkkaApplication startup
    # Options: ERROR, WARNING, INFO, DEBUG
    # stdout-loglevel = "WARNING"

core/src/main/scala/spark/util/AkkaUtils.scala

@@ -30,6 +30,7 @@ private[spark] object AkkaUtils {
     val akkaConf = ConfigFactory.parseString("""
       akka.daemonic = on
       akka.event-handlers = ["akka.event.slf4j.Slf4jEventHandler"]
+      akka.stdout-loglevel = "ERROR"
       akka.actor.provider = "akka.remote.RemoteActorRefProvider"
       akka.remote.transport = "akka.remote.netty.NettyRemoteTransport"
       akka.remote.log-remote-lifecycle-events = on