[SPARK-17002][CORE] Document that spark.ssl.protocol. is required for SSL

## What changes were proposed in this pull request?

`spark.ssl.enabled`=true, but failing to set `spark.ssl.protocol` will fail and throw meaningless exception. `spark.ssl.protocol` is required when `spark.ssl.enabled`.

Improvement: require `spark.ssl.protocol` when initializing SSLContext, otherwise throws an exception to indicate that.

Remove the OrElse("default").

Document this requirement in configure.md

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

Manual tests:
Build document and check document

Configure `spark.ssl.enabled` only, it throws exception below:
6/08/16 16:04:37 INFO SecurityManager: SecurityManager: authentication disabled; ui acls disabled; users  with view permissions: Set(mwang); groups with view permissions: Set(); users  with modify permissions: Set(mwang); groups with modify permissions: Set()
Exception in thread "main" java.lang.IllegalArgumentException: requirement failed: spark.ssl.protocol is required when enabling SSL connections.
	at scala.Predef$.require(Predef.scala:224)
	at org.apache.spark.SecurityManager.<init>(SecurityManager.scala:285)
	at org.apache.spark.deploy.master.Master$.startRpcEnvAndEndpoint(Master.scala:1026)
	at org.apache.spark.deploy.master.Master$.main(Master.scala:1011)
	at org.apache.spark.deploy.master.Master.main(Master.scala)

Configure `spark.ssl.protocol`  and `spark.ssl.protocol`
It works fine.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #14674 from wangmiao1981/ssl.

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

@@ -282,7 +282,10 @@ private[spark] class SecurityManager(sparkConf: SparkConf)
       }: TrustManager
     })
 
-    val sslContext = SSLContext.getInstance(fileServerSSLOptions.protocol.getOrElse("Default"))
+    require(fileServerSSLOptions.protocol.isDefined,
+      "spark.ssl.protocol is required when enabling SSL connections.")
+
+    val sslContext = SSLContext.getInstance(fileServerSSLOptions.protocol.get)
     sslContext.init(null, trustStoreManagers.getOrElse(credulousTrustStoreManagers), null)
 
     val hostVerifier = new HostnameVerifier {

docs/configuration.md

@@ -1472,6 +1472,9 @@ Apart from these, the following properties are also available, and may be useful
         <td>
             <p>Whether to enable SSL connections on all supported protocols.</p>
 
+            <p>When <code>spark.ssl.enabled</code> is configured, <code>spark.ssl.protocol</code>
+            is required.</p>
+
             <p>All the SSL settings like <code>spark.ssl.xxx</code> where <code>xxx</code> is a
             particular configuration property, denote the global configuration for all the supported
             protocols. In order to override the global configuration for the particular protocol,