Adding device and host tracking for intents (ONOS-1356)

Also, this should fix ONOS-1184 (intents submitted before hosts detected).

Change-Id: I47a503c18dc728912132eb2e2fcc160d47e518eb
diff --git a/core/api/src/main/java/org/onosproject/net/ElementId.java b/core/api/src/main/java/org/onosproject/net/ElementId.java
index 49e110b..9e4b3cd 100644
--- a/core/api/src/main/java/org/onosproject/net/ElementId.java
+++ b/core/api/src/main/java/org/onosproject/net/ElementId.java
@@ -18,5 +18,5 @@
 /**
  * Immutable representation of a network element identity.
  */
-public abstract class ElementId {
+public abstract class ElementId implements NetworkResource {
 }