Enable REST to manage devices under a proxy

This commit integrates the REST proxy within the standard REST
protocol and provider, supporting multiple devices managed by
a single REST end point (e.g. a REST-based second level controller).

It introduces the concept of RestSBServer, that represents
the REST service exposed by a single or a set of devices.

Change-Id: I0b187151848c20bc1fd8c39a33d57a500f667533
diff --git a/core/api/src/main/java/org/onosproject/net/AnnotationKeys.java b/core/api/src/main/java/org/onosproject/net/AnnotationKeys.java
index 5345e34..9e68636 100644
--- a/core/api/src/main/java/org/onosproject/net/AnnotationKeys.java
+++ b/core/api/src/main/java/org/onosproject/net/AnnotationKeys.java
@@ -156,6 +156,11 @@
     public static final String PROTECTED = "protected";
 
     /**
+     * Annotation key for REST server identifier.
+     */
+    public static final String REST_SERVER = "restServer";
+
+    /**
      * Returns the value annotated object for the specified annotation key.
      * The annotated value is expected to be String that can be parsed as double.
      * If parsing fails, the returned value will be 1.0.