KAFKA-4301; Add more trace for SSL handshake

Author: Rajini Sivaram <rajinisivaram@googlemail.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>

Closes #2027 from rajinisivaram/KAFKA-4301

clients/src/main/java/org/apache/kafka/common/network/SslTransportLayer.java

@@ -352,8 +352,12 @@ public class SslTransportLayer implements TransportLayer {
             //remove OP_WRITE if we are complete, otherwise we still have data to write
             if (!handshakeComplete)
                 key.interestOps(key.interestOps() | SelectionKey.OP_WRITE);
-            else
+            else {
                 key.interestOps(key.interestOps() & ~SelectionKey.OP_WRITE);
+                SSLSession session = sslEngine.getSession();
+                log.debug("SSL handshake completed successfully with peerHost '{}' peerPort {} peerPrincipal '{}' cipherSuite '{}'",
+                        session.getPeerHost(), session.getPeerPort(), peerPrincipal(), session.getCipherSuite());
+            }
 
             log.trace("SSLHandshake FINISHED channelId {}, appReadBuffer pos {}, netReadBuffer pos {}, netWriteBuffer pos {} ",
                       channelId, appReadBuffer.position(), netReadBuffer.position(), netWriteBuffer.position());