slapos_pdm: software_release is an optional variation

We can not update the Open Order every time one Instance is upgraded

master/bt5/slapos_pdm/DocumentTemplateItem/portal_components/document.erp5_slapos.SoftwareProduct.py

@@ -57,6 +57,7 @@ class SoftwareProduct(ERP5SoftwareProduct):
   content_type = ''
 
   variation_base_category_list = ('software_release', 'software_type', )
+  optional_variation_base_category_list = ('software_release', )
 
   default_category_list = (
     'base_contribution/base_amount/invoicing/discounted',