fix parsing of @bind (instead of @nodeset) on xform inputs

Fixes pact form data 500

```
  File "/home/cchq/www/production/code_root/corehq/apps/app_manager/xform.py", line 769, in get_path
    raise BindNotFound('No binding found for %s' % bind_id)

BindNotFound: No binding found for clients_phone_number_1
```