[SPARK-5724] fix the misconfiguration in AkkaUtils

https://issues.apache.org/jira/browse/SPARK-5724

In AkkaUtil, we set several failure detector related the parameters as following

```
al akkaConf = ConfigFactory.parseMap(conf.getAkkaConf.toMap[String, String])
      .withFallback(akkaSslConfig).withFallback(ConfigFactory.parseString(
      s"""
      |akka.daemonic = on
      |akka.loggers = [""akka.event.slf4j.Slf4jLogger""]
      |akka.stdout-loglevel = "ERROR"
      |akka.jvm-exit-on-fatal-error = off
      |akka.remote.require-cookie = "$requireCookie"
      |akka.remote.secure-cookie = "$secureCookie"
      |akka.remote.transport-failure-detector.heartbeat-interval = $akkaHeartBeatInterval s
      |akka.remote.transport-failure-detector.acceptable-heartbeat-pause = $akkaHeartBeatPauses s
      |akka.remote.transport-failure-detector.threshold = $akkaFailureDetector
      |akka.actor.provider = "akka.remote.RemoteActorRefProvider"
      |akka.remote.netty.tcp.transport-class = "akka.remote.transport.netty.NettyTransport"
      |akka.remote.netty.tcp.hostname = "$host"
      |akka.remote.netty.tcp.port = $port
      |akka.remote.netty.tcp.tcp-nodelay = on
      |akka.remote.netty.tcp.connection-timeout = $akkaTimeout s
      |akka.remote.netty.tcp.maximum-frame-size = ${akkaFrameSize}B
      |akka.remote.netty.tcp.execution-pool-size = $akkaThreads
      |akka.actor.default-dispatcher.throughput = $akkaBatchSize
      |akka.log-config-on-start = $logAkkaConfig
      |akka.remote.log-remote-lifecycle-events = $lifecycleEvents
      |akka.log-dead-letters = $lifecycleEvents
      |akka.log-dead-letters-during-shutdown = $lifecycleEvents
      """.stripMargin))

```

Actually, we do not have any parameter naming "akka.remote.transport-failure-detector.threshold"
see: http://doc.akka.io/docs/akka/2.3.4/general/configuration.html
what we have is "akka.remote.watch-failure-detector.threshold"

Author: CodingCat <zhunansjtu@gmail.com>

Closes #4512 from CodingCat/SPARK-5724 and squashes the following commits:

bafe56e [CodingCat] fix the grammar in configuration doc
338296e [CodingCat] remove failure-detector related info
8bfcfd4 [CodingCat] fix the misconfiguration in AkkaUtils