Adding REST API and CLI for managing UI user preferences.

Change-Id: If5a1b347bcf7443120c2dfbb096fca6695b8f0e2
diff --git a/web/gui/src/main/java/org/onosproject/ui/impl/UiExtensionManager.java b/web/gui/src/main/java/org/onosproject/ui/impl/UiExtensionManager.java
index f3afa1a..a50fd1b 100644
--- a/web/gui/src/main/java/org/onosproject/ui/impl/UiExtensionManager.java
+++ b/web/gui/src/main/java/org/onosproject/ui/impl/UiExtensionManager.java
@@ -366,7 +366,11 @@
 
     @Override
     public void setPreference(String username, String key, ObjectNode value) {
-        prefs.put(key(username, key), value);
+        if (value != null) {
+            prefs.put(key(username, key), value);
+        } else {
+            prefs.remove(key(username, key));
+        }
     }
 
     // =====================================================================