[SPARK-18353][CORE] spark.rpc.askTimeout defalut value is not 120s

## What changes were proposed in this pull request?

Avoid hard-coding spark.rpc.askTimeout to non-default in Client; fix doc about spark.rpc.askTimeout default

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #15833 from srowen/SPARK-18353.

core/src/main/scala/org/apache/spark/deploy/Client.scala

@@ -221,7 +221,9 @@ object Client {
     val conf = new SparkConf()
     val driverArgs = new ClientArguments(args)
 
-    conf.set("spark.rpc.askTimeout", "10")
+    if (!conf.contains("spark.rpc.askTimeout")) {
+      conf.set("spark.rpc.askTimeout", "10s")
+    }
     Logger.getRootLogger.setLevel(driverArgs.logLevel)
 
     val rpcEnv =

docs/configuration.md

@@ -1184,7 +1184,7 @@ Apart from these, the following properties are also available, and may be useful
 </tr>
 <tr>
   <td><code>spark.rpc.askTimeout</code></td>
-  <td>120s</td>
+  <td><code>spark.network.timeout</code></td>
   <td>
     Duration for an RPC ask operation to wait before timing out.
   </td>
@@ -1566,7 +1566,7 @@ Apart from these, the following properties are also available, and may be useful
 </tr>
 <tr>
   <td><code>spark.core.connection.ack.wait.timeout</code></td>
-  <td>60s</td>
+  <td><code>spark.network.timeout</code></td>
   <td>
     How long for the connection to wait for ack to occur before timing
     out and giving up. To avoid unwilling timeout caused by long pause like GC,