OS Design: invert new and old name

Internally in ganeti we already have renamed the instance, so it's
better to pass the new name in INSTANCE_NAME and the old one in
OLD_INSTANCE_NAME rather than the old name in INSTANCE_NAME and the new
one in NEW_INSTANCE_NAME as this reflects what the cluster thinks.

Reviewed-by: iustinp

doc/design-2.0-os-interface.rst

@@ -160,8 +160,8 @@ page. All these variables will be available to all scripts.
 Some scripts will need a few more information to work. These will have
 per-script variables, such as for example:
 
-NEW_INSTANCE_NAME
-  rename: the name the instance should be renamed to.
+OLD_INSTANCE_NAME
+  rename: the name the instance should be renamed from.
 EXPORT_NODE
   import/export: node where the export should be saved to or sourced from.
 EXPORT_PATH