AETHER-3189 Update ids to be dns compliant:

Change-Id: Ic03b51c62fb205a5505a15e743f93b0057f9b93b
diff --git a/config-models/aether-2.0.x/files/yang/onf-upf.yang b/config-models/aether-2.0.x/files/yang/onf-upf.yang
index 72a7ec9..fdfbafc 100755
--- a/config-models/aether-2.0.x/files/yang/onf-upf.yang
+++ b/config-models/aether-2.0.x/files/yang/onf-upf.yang
@@ -6,7 +6,6 @@
   belongs-to onf-enterprise { prefix ent; }
 
   import ietf-inet-types { prefix inet; }
-  import ietf-yang-types{ prefix yg; }
   import onf-aether-types { prefix at; }
 
   organization "Open Networking Foundation.";
@@ -24,10 +23,8 @@
   }
 
   typedef upf-id {
-        type yg:yang-identifier {
-            length 1..32;
-        }
-   description "The typedef for upf-id";
+    type at:aether-identifier;
+    description "The typedef for upf-id";
   }
 
   grouping upf {