Supply username for compute node admin account in inventory

Change-Id: Ie6a82a6eca975c1ccadb101fd1f46dc28e3128f1
diff --git a/roles/head-node/templates/pod-inventory b/roles/head-node/templates/pod-inventory
index 89c368f..3b8d6bb 100644
--- a/roles/head-node/templates/pod-inventory
+++ b/roles/head-node/templates/pod-inventory
@@ -22,7 +22,8 @@
 fi
 
 cat << EO_TAIL
-    ]
+    ],
+    "vars" : { "ansible_user": "ubuntu" }
   }
 }
 EO_TAIL