Fix RDDOperationScope serialization issues (apache-spark-on-k8s#379)

Follow up to jackson 2.9.5 upgrade where RDDOperationScope can't find the serializer for the Option (parent variable) and serializes it using defaults. This force jackson to use static typing, i.e. use the class types and not values determined using reflection at runtime

Author: robert3005

core/src/main/scala/org/apache/spark/rdd/RDDOperationScope.scala

@@ -22,6 +22,7 @@ import java.util.concurrent.atomic.AtomicInteger
 import com.fasterxml.jackson.annotation.{JsonIgnore, JsonInclude, JsonPropertyOrder}
 import com.fasterxml.jackson.annotation.JsonInclude.Include
 import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.databind.annotation.JsonSerialize
 import com.fasterxml.jackson.module.scala.DefaultScalaModule
 import com.google.common.base.Objects
 
@@ -45,6 +46,7 @@ import org.apache.spark.internal.Logging
 @JsonPropertyOrder(Array("id", "name", "parent"))
 private[spark] class RDDOperationScope(
     val name: String,
+    @JsonSerialize(typing = JsonSerialize.Typing.STATIC)
     val parent: Option[RDDOperationScope] = None,
     val id: String = RDDOperationScope.nextScopeId().toString) {