[SPARK-5530] Add executor container to executorIdToContainer

when call killExecutor method, it will only go to the else branch, because  the variable executorIdToContainer never be put any value.

Author: Xutingjun <1039320815@qq.com>

Closes #4309 from XuTingjun/dynamicAllocator and squashes the following commits:

c823418 [Xutingjun] fix bugwq

yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocator.scala

@@ -300,6 +300,7 @@ private[yarn] class YarnAllocator(
       assert(container.getResource.getMemory >= resource.getMemory)
 
       logInfo("Launching container %s for on host %s".format(containerId, executorHostname))
+      executorIdToContainer(executorId) = container      
 
       val containerSet = allocatedHostToContainersMap.getOrElseUpdate(executorHostname,
         new HashSet[ContainerId])