switch to ImmutableList

Change-Id: Ia5f8936d0aaf447253f5dde0db1ee688441d2472
diff --git a/core/api/src/main/java/org/onlab/onos/cluster/RoleInfo.java b/core/api/src/main/java/org/onlab/onos/cluster/RoleInfo.java
index d2bee8b..45b96ab 100644
--- a/core/api/src/main/java/org/onlab/onos/cluster/RoleInfo.java
+++ b/core/api/src/main/java/org/onlab/onos/cluster/RoleInfo.java
@@ -1,9 +1,10 @@
 package org.onlab.onos.cluster;
 
-import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 
+import com.google.common.collect.ImmutableList;
+
 /**
  * A container for detailed role information for a device,
  * within the current cluster. Role attributes include current
@@ -15,7 +16,7 @@
 
     public RoleInfo(NodeId master, List<NodeId> backups) {
         this.master = master;
-        this.backups = Collections.unmodifiableList(backups);
+        this.backups = ImmutableList.copyOf(backups);
     }
 
     public NodeId master() {