elasticsearch: Support native elasticsearch config for transport mode.

Previously, this feature was available only for node mode.
By supporting loading native elasticsearch config, it is possible now
to use Shield with syslog-ng (even LDAP is working).

When you want to use Shield, you have to add elasticsearch-shield-1.3.3.jar
to your classpath.

```
shield.ssl.keystore.path: /home/test/es/shield/client.jks
shield.ssl.keystore.password: qqq123
shield.transport.ssl: true
shield.user: es_admin:qqq123
```

```
@Version: 3.8

destination d_elastic {
    elasticsearch(
        client_lib_dir(/usr/share/elasticsearch/lib)
        client_mode("transport")
        cluster("es-syslog-ng")
        index("es-syslog-ng")
        port("9300")
        server("vagrant-es-server")
        type("slng_test_type")
        resource("/opt/syslog-ng/etc/elasticsearch.yml")
    );
};

```

fixes: #826

Signed-off-by: Laszlo Budai <Laszlo.Budai@balabit.com>

modules/java-modules/elastic/src/main/java/org/syslog_ng/elasticsearch/client/ESClient.java

@@ -23,12 +23,20 @@
 
 package org.syslog_ng.elasticsearch.client;
 
+import java.io.File;
+import java.net.URI;
+import java.net.URL;
+import java.net.MalformedURLException;
+import java.nio.file.Paths;
+
 import org.apache.log4j.Logger;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthRequestBuilder;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthResponse;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthStatus;
 import org.elasticsearch.client.Client;
+import org.elasticsearch.common.settings.ImmutableSettings.Builder;
+import org.elasticsearch.common.settings.SettingsException;
 import org.syslog_ng.elasticsearch.ElasticSearchOptions;
 
 public abstract class ESClient {
@@ -104,4 +112,18 @@ public Client getClient() {
 	protected void resetClient() {
 		this.client = null;
 	}
+
+	protected void loadConfigFile(String cfgFile, Builder settingsBuilder) {
+		if (cfgFile == null || cfgFile.isEmpty()) {
+			return;
+		}
+		try {
+			URL url = new File(cfgFile).toURI().toURL();
+			settingsBuilder.loadFromUrl(url);
+		} catch (MalformedURLException e) {
+			logger.warn("Bad filename format, filename = '" + cfgFile + "'");
+		} catch (SettingsException e) {
+			logger.warn("Can't load settings from file, file = '" + cfgFile + "', reason = '" + e.getMessage() + "'");
+		}
+	}
 }

modules/java-modules/elastic/src/main/java/org/syslog_ng/elasticsearch/client/ESNodeClient.java

@@ -59,26 +59,11 @@ private NodeBuilder createNodeBuilder(String cluster) {
 		return result;
 	}
 
-	private void loadConfigFile(String cfgFile, NodeBuilder nodeBuilder) {
-		if (cfgFile == null || cfgFile.isEmpty()) {
-			return;
-		}
-		try {
-			URL url = new File(cfgFile).toURI().toURL();
-			Builder builder = nodeBuilder().settings().loadFromUrl(url);
-			nodeBuilder = nodeBuilder.settings(builder);
-		} catch (MalformedURLException e) {
-			logger.warn("Bad filename format, filename = '" + cfgFile + "'");
-		} catch (SettingsException e) {
-			logger.warn("Can't load settings from file, file = '" + cfgFile + "', reason = '" + e.getMessage() + "'");
-		}
-	}
-
 	@Override
 	public Client createClient() {
 		NodeBuilder nodeBuilder = createNodeBuilder(options.getCluster());
 		nodeBuilder.settings().put("discovery.initial_state_timeout", "5s");
-		loadConfigFile(options.getConfigFile(), nodeBuilder);
+		loadConfigFile(options.getConfigFile(), nodeBuilder.settings());
 		node = nodeBuilder.node();
 	    return node.client();
 	}

modules/java-modules/elastic/src/main/java/org/syslog_ng/elasticsearch/client/ESTransportClient.java

@@ -38,17 +38,23 @@ public class ESTransportClient extends ESClient {
 	public ESTransportClient(ElasticSearchOptions options) {
 		super(options);
 	}
-
-	public Client createClient() {
-		String clusterName = options.getCluster();
+
+	private Settings buildSettings() {
 		Builder settingsBuilder = ImmutableSettings.settingsBuilder();
+		String clusterName = options.getCluster();
+
+		loadConfigFile(options.getConfigFile(), settingsBuilder);
 		settingsBuilder = settingsBuilder.put("client.transport.sniff", true).classLoader(Settings.class.getClassLoader());
 
 		if (clusterName != null) {
-			settingsBuilder = settingsBuilder.put("cluster.name", clusterName);
+		        settingsBuilder = settingsBuilder.put("cluster.name", clusterName);
 		}
 
-		settings = settingsBuilder.build();
+		return settingsBuilder.build();
+	}
+
+	public Client createClient() {
+		settings = buildSettings();
 
 		String[] servers =  options.getServerList();