[LIVY-697] Rsc client cannot resolve the hostname of driver in yarn-c…

…luster mode

## What changes were proposed in this pull request?

[LIVY-697] Rsc client cannot resolve the hostname of driver in yarn-cluster mode

1. The content of Driver in /etc/hosts are as follows:
   127.0.0.1 localhost
   10.10.10.10 test_hostname

2. The content of Driver in /etc/hostname are as follows:
   test_hostname

3. The findLocalAddress method in livy cannot return 10.10.10.10, but return  test_hostname.
   Because the  test_hostname point to 10.10.10.10,  which doesn't pass the check
   address.isLoopbackAddress(), so findLocalAddress return test_hostname .

4. The rsc client cannot resolve the test_hostname, which cause rsc client cannot connect to
   driver.

5. The findLocalAddress method in livy can return 10.10.10.10 as expected if the content
   of Driver in  /etc/hosts are as follows, which is not correct in our environment.
   127.0.0.1 localhost
   127.0.0.1 test_hostname

6. Though I can modify the findLocalAddress method to return 10.10.10.10, but it maybe cause
  error if the machine has multiple  network  cards.  So, rsc client gets the driver ip from the
  connection.

## How was this patch tested?

1. The content of Driver in /etc/hosts are as follows:
   127.0.0.1 localhost
   127.0.0.1 test_hostname

2. The content of Driver in /etc/hostname are as follows:
   test_hostname

3. rsc client can get the driver ip in connection, and connect to driver successfully.

Author: runzhiwang <runzhiwang@tencent.com>

Closes #246 from runzhiwang/hostname-2-ip.

rsc/src/main/java/org/apache/livy/rsc/ContextLauncher.java

@@ -27,6 +27,7 @@
 import java.io.OutputStreamWriter;
 import java.io.Reader;
 import java.io.Writer;
+import java.net.InetSocketAddress;
 import java.nio.file.Files;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -340,7 +341,9 @@ void dispose() {
     }
 
     private void handle(ChannelHandlerContext ctx, RemoteDriverAddress msg) {
-      ContextInfo info = new ContextInfo(msg.host, msg.port, clientId, secret);
+      InetSocketAddress insocket = (InetSocketAddress) ctx.channel().remoteAddress();
+      String ip = insocket.getAddress().getHostAddress();
+      ContextInfo info = new ContextInfo(ip, msg.port, clientId, secret);
       if (promise.trySuccess(info)) {
         timeout.cancel(true);
         LOG.debug("Received driver info for client {}: {}/{}.", client.getChannel(),