Fix message preventing overwrite the instance_uuid

If a node already has an instance associated with it, we prevent
the instance_uuid from being updated (one should tear-down the node
first). When trying to update the instance_uuid the message returned
was saying that the node was associated with the new instance value,
this patch fix the message to return the current instance_uuid associated
with the node.

Change-Id: Ib8ab6c96b28b49b6259a95e82c83c5fbdeba5f91
Closes-Bug: #1308144
Merge "Fix message preventing overwrite the instance_uuid"