Add string attribute interfaces.

- Add interface definition to be used for UC1. (ONOS-1428, ONOS-1354, ONOS-1390)
- Added toString which was missing only on SwitchImpl.

Change-Id: I5a97d5132129213d78999e4f24917e5e5dbe4496
diff --git a/src/main/java/net/onrc/onos/core/topology/Link.java b/src/main/java/net/onrc/onos/core/topology/Link.java
index 768d290..f1fcae8 100644
--- a/src/main/java/net/onrc/onos/core/topology/Link.java
+++ b/src/main/java/net/onrc/onos/core/topology/Link.java
@@ -10,7 +10,7 @@
  * Interface of Link object in the topology.
  */
 @JsonSerialize(using = LinkSerializer.class)
-public interface Link {
+public interface Link extends StringAttributes {
     /**
      * Gets the source switch for the link.
      *