remove default config value (#781)

diff --git a/dubbo-admin-server/src/main/java/org/apache/dubbo/admin/config/ConfigCenter.java b/dubbo-admin-server/src/main/java/org/apache/dubbo/admin/config/ConfigCenter.java
index 43a2dc1..bedd84d 100644
--- a/dubbo-admin-server/src/main/java/org/apache/dubbo/admin/config/ConfigCenter.java
+++ b/dubbo-admin-server/src/main/java/org/apache/dubbo/admin/config/ConfigCenter.java
@@ -56,22 +56,22 @@
     @Value("${admin.metadata-report.cluster:false}")
     private boolean cluster;
 
-    @Value("${admin.registry.group:dubbo}")
+    @Value("${admin.registry.group:}")
     private String registryGroup;
 
-    @Value("${admin.config-center.group:dubbo}")
+    @Value("${admin.config-center.group:}")
     private String configCenterGroup;
 
-    @Value("${admin.metadata-report.group:dubbo}")
+    @Value("${admin.metadata-report.group:}")
     private String metadataGroup;
 
-    @Value("${admin.registry.namespace:dubbo}")
+    @Value("${admin.registry.namespace:}")
     private String registryNameSpace;
 
-    @Value("${admin.config-center.namespace:dubbo}")
+    @Value("${admin.config-center.namespace:}")
     private String configCenterGroupNameSpace;
 
-    @Value("${admin.metadata-report.namespace:dubbo}")
+    @Value("${admin.metadata-report.namespace:}")
     private String metadataGroupNameSpace;
 
     @Value("${admin.config-center.username:}")