add toString

Change-Id: I14b07972b1f0aec1a7cc21ff053aa30a25af8ca1

@@ -4,6 +4,8 @@ import java.util.Objects;
 
 import org.onlab.onos.cluster.NodeId;
 
+import com.google.common.base.MoreObjects;
+
 public final class MastershipTerm {
 
     private final NodeId master;
@@ -48,4 +50,12 @@ public final class MastershipTerm {
         }
         return false;
     }
+
+    @Override
+    public String toString() {
+        return MoreObjects.toStringHelper(getClass())
+                .add("master", this.master)
+                .add("termNumber", this.termNumber)
+                .toString();
+    }
 }