slapos_subscription_request: use souce_project to find the trade condition

no need to trick source/destination project anymore, as trade_condition_type is used

master/bt5/slapos_subscription_request/SkinTemplateItem/portal_skins/slapos_subscription_request/Resource_createSubscriptionRequest.py

@@ -17,7 +17,7 @@ elif resource.getPortalType() == "Service":
   if resource.getRelativeUrl() == "service_module/slapos_compute_node_subscription":
     if project_value is None:
       raise AssertionError('A project is required for %s %s' % (resource.getRelativeUrl(), project_value))
-    destination_project_value = project_value
+    source_project_value = project_value
 
     trade_condition_type = "compute_node"
   elif resource.getRelativeUrl() == "service_module/slapos_virtual_master_subscription":