Change the name of input ragument in ClusterScheduler#initialize from context to backend.

The SchedulerBackend used to be called ClusterSchedulerContext so just want to make small
change of the input param in the ClusterScheduler#initialize to reflect this.

core/src/main/scala/org/apache/spark/scheduler/cluster/ClusterScheduler.scala

@@ -100,8 +100,8 @@ private[spark] class ClusterScheduler(val sc: SparkContext)
     this.dagScheduler = dagScheduler
   }
 
-  def initialize(context: SchedulerBackend) {
-    backend = context
+  def initialize(backend: SchedulerBackend) {
+    this.backend = backend
     // temporarily set rootPool name to empty
     rootPool = new Pool("", schedulingMode, 0, 0)
     schedulableBuilder = {