SONAR suggestion - remove mutable public collections

Change-Id: If2980109c42399f566805a5b741cf7e10ab6544e
diff --git a/apps/gluon/src/main/java/org/onosproject/gluon/manager/GluonManager.java b/apps/gluon/src/main/java/org/onosproject/gluon/manager/GluonManager.java
index 11f3738..c0a4b67 100644
--- a/apps/gluon/src/main/java/org/onosproject/gluon/manager/GluonManager.java
+++ b/apps/gluon/src/main/java/org/onosproject/gluon/manager/GluonManager.java
@@ -57,7 +57,7 @@
                 }
             };
 
-    public static Map<String, GluonServer> serverMap = new LinkedHashMap<>();
+    private static Map<String, GluonServer> serverMap = new LinkedHashMap<>();
 
     @Activate
     public void activate() {