Better shield user configuration

Added `shield.user` setting so that the clients won't need to go through the unnatural and tedious process of configuring the `Authorization` header directly (that also requires the user to applicat the base64(username:password) logic.

Now, the user can just set the following settings to bind a user to the client:

```yaml
shield.user: 'username:password'
```

Original commit: elastic/x-pack-elasticsearch@94be3abd92

src/main/java/org/elasticsearch/shield/plugin/ShieldPlugin.java

@@ -5,11 +5,17 @@
  */
 package org.elasticsearch.shield.plugin;
 
+import org.elasticsearch.client.support.Headers;
 import org.elasticsearch.common.collect.ImmutableList;
 import org.elasticsearch.common.inject.Module;
+import org.elasticsearch.common.settings.ImmutableSettings;
+import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.env.Environment;
 import org.elasticsearch.plugins.AbstractPlugin;
 import org.elasticsearch.shield.ShieldModule;
+import org.elasticsearch.shield.ShieldSettingsException;
+import org.elasticsearch.shield.authc.support.SecuredString;
+import org.elasticsearch.shield.authc.support.UsernamePasswordToken;
 
 import java.io.File;
 import java.nio.file.Path;
@@ -22,6 +28,12 @@ public class ShieldPlugin extends AbstractPlugin {
 
     public static final String NAME = "shield";
 
+    private final Settings settings;
+
+    public ShieldPlugin(Settings settings) {
+        this.settings = settings;
+    }
+
     @Override
     public String name() {
         return NAME;
@@ -37,6 +49,26 @@ public class ShieldPlugin extends AbstractPlugin {
         return ImmutableList.<Class<? extends Module>>of(ShieldModule.class);
     }
 
+    @Override
+    public Settings additionalSettings() {
+        String setting = Headers.PREFIX + "." + UsernamePasswordToken.BASIC_AUTH_HEADER;
+        if (settings.get(setting) != null) {
+            return ImmutableSettings.EMPTY;
+        }
+        String user = settings.get("shield.user");
+        if (user == null) {
+            return ImmutableSettings.EMPTY;
+        }
+        int i = user.indexOf(":");
+        if (i < 0 || i == user.length() - 1) {
+            throw new ShieldSettingsException("Invalid [shield.user] settings. Must be in the form of \"<username>:<password>\"");
+        }
+        String username = user.substring(0, i);
+        String password = user.substring(i + 1);
+        return ImmutableSettings.builder()
+                .put(setting, UsernamePasswordToken.basicAuthHeaderValue(username, new SecuredString(password.toCharArray()))).build();
+    }
+
     public static Path configDir(Environment env) {
         return new File(env.configFile(), NAME).toPath();
     }

src/test/java/org/elasticsearch/shield/test/ShieldIntegrationTest.java

@@ -63,7 +63,6 @@ public abstract class ShieldIntegrationTest extends ElasticsearchIntegrationTest
         File folder = newFolder();
 
         ImmutableSettings.Builder builder = ImmutableSettings.builder()
-                .put("request.headers.Authorization", basicAuthHeaderValue(getClientUsername(), getClientPassword()))
                 .put("discovery.zen.ping.multicast.enabled", false)
                 .put("discovery.type", "zen")
                 .put("node.mode", "network")
@@ -76,6 +75,8 @@ public abstract class ShieldIntegrationTest extends ElasticsearchIntegrationTest
                 .put("shield.audit.enabled", true)
                 .put("plugins.load_classpath_plugins", false);
 
+        setUser(builder);
+
         if (OsUtils.MAC) {
             builder.put("network.host", randomBoolean() ? "127.0.0.1" : "::1");
         }
@@ -97,13 +98,24 @@ public abstract class ShieldIntegrationTest extends ElasticsearchIntegrationTest
 
     @Override
     protected Settings transportClientSettings() {
-        return ImmutableSettings.builder()
+        ImmutableSettings.Builder builder = ImmutableSettings.builder()
                 .put("request.headers.Authorization", basicAuthHeaderValue(getClientUsername(), getClientPassword()))
                 .put(TransportModule.TRANSPORT_TYPE_KEY, NettySecuredTransport.class.getName())
                 .put("plugins." + PluginsService.LOAD_PLUGIN_FROM_CLASSPATH, false)
                 .put("node.mode", "network")
-                .put(getSSLSettingsForStore("/org/elasticsearch/shield/transport/ssl/certs/simple/testclient.jks", "testclient"))
-                .build();
+                .put(getSSLSettingsForStore("/org/elasticsearch/shield/transport/ssl/certs/simple/testclient.jks", "testclient"));
+
+        setUser(builder);
+
+        return builder.build();
+    }
+
+    protected void setUser(ImmutableSettings.Builder settings) {
+        if (randomBoolean()) {
+            settings.put("request.headers.Authorization", basicAuthHeaderValue(getClientUsername(), getClientPassword()));
+        } else {
+            settings.put("shield.user", getClientUsername() + ":" + new String(getClientPassword().internalChars()));
+        }
     }
 
     protected String writeFile(File folder, String name, String content) {