MastershipService.relinquishRole returns CompletableFuture + Block deviceDiconnected until role relinquish is complete

Change-Id: I081df48fc05fdca2e452a937a093d5caa16091ed
diff --git a/core/net/src/test/java/org/onosproject/net/device/impl/DeviceManagerTest.java b/core/net/src/test/java/org/onosproject/net/device/impl/DeviceManagerTest.java
index 0b6c3e0..fdda578 100644
--- a/core/net/src/test/java/org/onosproject/net/device/impl/DeviceManagerTest.java
+++ b/core/net/src/test/java/org/onosproject/net/device/impl/DeviceManagerTest.java
@@ -311,6 +311,11 @@
         }
 
         @Override
+        public CompletableFuture<Void> relinquishMastership(DeviceId deviceId) {
+            return CompletableFuture.completedFuture(null);
+        }
+
+        @Override
         public MastershipTerm getMastershipTerm(DeviceId deviceId) {
             // FIXME: just returning something not null
             return MastershipTerm.of(NID_LOCAL, 1);