Added ability to force mastership re-balancing between instances from the GUI.

Change-Id: I98e56deb3e2b00df630ed85b596c8e35b3d6efab
diff --git a/core/api/src/main/java/org/onlab/onos/mastership/MastershipAdminService.java b/core/api/src/main/java/org/onlab/onos/mastership/MastershipAdminService.java
index f2fd8f4..cb3869d 100644
--- a/core/api/src/main/java/org/onlab/onos/mastership/MastershipAdminService.java
+++ b/core/api/src/main/java/org/onlab/onos/mastership/MastershipAdminService.java
@@ -33,4 +33,10 @@
      */
     void setRole(NodeId instance, DeviceId deviceId, MastershipRole role);
 
+    /**
+     * Balances the mastership to be shared as evenly as possibly by all
+     * online instances.
+     */
+    void balanceRoles();
+
 }