Relinquish mastership, tests, and few modifications to trivial MastershipStore

Change-Id: Iae29de010f13cb3ee02bcb316510cc254d5756fc
diff --git a/core/store/src/main/java/org/onlab/onos/store/cluster/impl/DistributedMastershipStore.java b/core/store/src/main/java/org/onlab/onos/store/cluster/impl/DistributedMastershipStore.java
index 46bf6af..00d0547 100644
--- a/core/store/src/main/java/org/onlab/onos/store/cluster/impl/DistributedMastershipStore.java
+++ b/core/store/src/main/java/org/onlab/onos/store/cluster/impl/DistributedMastershipStore.java
@@ -122,4 +122,10 @@
         return null;
     }
 
+    @Override
+    public MastershipEvent unsetMaster(NodeId nodeId, DeviceId deviceId) {
+        // TODO Auto-generated method stub
+        return null;
+    }
+
 }