Fix sst definition for slave

Change-Id: I90d7f0ee195dc16aecb49aa99ed8256b6df09d4c
diff --git a/metadata/service/slave/cluster.yml b/metadata/service/slave/cluster.yml
index a4c7133..991db2a 100644
--- a/metadata/service/slave/cluster.yml
+++ b/metadata/service/slave/cluster.yml
@@ -29,7 +29,7 @@
       admin:
         user: ${_param:galera_server_admin_user}
         password: ${_param:galera_server_admin_password}
-      sst_user:
+      sst:
         user: ${_param:galera_server_sst_user}
         password: ${_param:galera_server_sst_password}
       members: