fix(config-etcd): 修复 EtcdConfig 默认连接字符串并统一命名风格

- 将 EtcdConfig 类中的属性名称从连字符风格改为驼峰风格
- 修改默认连接字符串从 ZooKeeper 地址改为 Etcd 默认地址
- 在 demo-etcd 的 pom 文件中添加新的配置属性

Author: SweetWuXiaoMei

demo/demo-etcd/test-client/pom.xml

@@ -128,6 +128,7 @@
                       <env>
                         <JAVA_OPTS>
                           -Dservicecomb.registry.etcd.connectString=http://etcd:2379
+                          -Dservicecomb.config.etcd.connectString=http://etcd:2379
                         </JAVA_OPTS>
                         <JAR_PATH>/maven/maven/etcd-consumer-${project.version}.jar</JAR_PATH>
                       </env>

dynamic-config/config-etcd/src/main/java/org/apache/servicecomb/config/etcd/EtcdConfig.java

@@ -21,17 +21,17 @@
 public class EtcdConfig {
   public static final String ZOOKEEPER_DEFAULT_ENVIRONMENT = "production";
 
-  public static final String PROPERTY_CONNECT_STRING = "servicecomb.config.etcd.connect-string";
+  public static final String PROPERTY_CONNECT_STRING = "servicecomb.config.etcd.connectString";
 
-  public static final String PROPERTY_SESSION_TIMEOUT = "servicecomb.config.etcd.session-timeout-millis";
+  public static final String PROPERTY_SESSION_TIMEOUT = "servicecomb.config.etcd.sessionTimeoutMillis";
 
-  public static final String PROPERTY_CONNECTION_TIMEOUT = "servicecomb.config.etcd.connection-timeout-mills";
+  public static final String PROPERTY_CONNECTION_TIMEOUT = "servicecomb.config.etcd.connectionTimeoutMills";
 
-  public static final String PROPERTY_AUTH_SCHEMA = "servicecomb.config.etcd.authentication-schema";
+  public static final String PROPERTY_AUTH_SCHEMA = "servicecomb.config.etcd.authenticationSchema";
 
-  public static final String PROPERTY_AUTH_INFO = "servicecomb.config.etcd.authentication-info";
+  public static final String PROPERTY_AUTH_INFO = "servicecomb.config.etcd.authenticationInfo";
 
-  public static final String PROPERTY_INSTANCE_TAG = "servicecomb.config.etcd.instance-tag";
+  public static final String PROPERTY_INSTANCE_TAG = "servicecomb.config.etcd.instanceTag";
 
   private final Environment environment;
 
@@ -40,7 +40,7 @@ public EtcdConfig(Environment environment) {
   }
 
   public String getConnectString() {
-    return environment.getProperty(PROPERTY_CONNECT_STRING, "http://127.0.0.1:2181");
+    return environment.getProperty(PROPERTY_CONNECT_STRING, "http://127.0.0.1:2379");
   }
 
   public int getSessionTimeoutMillis() {