Support connect to non-auth mongo server (#179)

diff --git a/scripts/start.sh b/scripts/start.sh
index 59571ef..0b390bf 100755
--- a/scripts/start.sh
+++ b/scripts/start.sh
@@ -56,9 +56,15 @@
 
 logBackupCount: 7
 EOM
+
+local uri="mongodb://${MONGODB_ADDR}/kie"
+if [ -n "${MONGODB_USER}" ]; then
+  uri="mongodb://${MONGODB_USER}:${MONGODB_PWD}@${MONGODB_ADDR}/kie"
+fi
+
 cat <<EOM > /etc/servicecomb-kie/kie-conf.yaml
 db:
-  uri: mongodb://${MONGODB_USER}:${MONGODB_PWD}@${MONGODB_ADDR}/kie
+  uri: ${uri}
   type: mongodb
   poolSize: 10
   ssl: false