Change ProviderId.NONE to none:none

Change-Id: I556f5cdbb555d195950985cdbd4dab538132a154
diff --git a/core/api/src/main/java/org/onlab/onos/net/provider/ProviderId.java b/core/api/src/main/java/org/onlab/onos/net/provider/ProviderId.java
index 2d29b52..e35d044 100644
--- a/core/api/src/main/java/org/onlab/onos/net/provider/ProviderId.java
+++ b/core/api/src/main/java/org/onlab/onos/net/provider/ProviderId.java
@@ -38,7 +38,7 @@
     /**
      * Represents no provider ID.
      */
-    public static final ProviderId NONE = new ProviderId();
+    public static final ProviderId NONE = new ProviderId("none", "none");
 
     private final String scheme;
     private final String id;