[SPARK-11501][CORE][YARN] Propagate spark.rpc config to executors

spark.rpc is supposed to be configurable but is not currently (doesn't get propagated to executors because RpcEnv.create is done before driver properties are fetched).

Author: Nishkam Ravi <nishkamravi@gmail.com>

Closes #9460 from nishkamravi2/master_akka.

core/src/main/scala/org/apache/spark/SparkConf.scala

@@ -629,6 +629,7 @@ private[spark] object SparkConf extends Logging {
     name.startsWith("spark.akka") ||
     (name.startsWith("spark.auth") && name != SecurityManager.SPARK_AUTH_SECRET_CONF) ||
     name.startsWith("spark.ssl") ||
+    name.startsWith("spark.rpc") ||
     isSparkPortConf(name)
   }