Fix UM2 upgrade regarding the variant

CURA-4482

UM2 by default doesn't have variants, but if the user enables Olsson
Block, the variant option will become available. This commit fixes the
following cases:

- Make sure that the variant is set on the extruder stack but not the
  global stack
- Extruder stacks don't contain information such as has_variant. Such
  info should be retrieved from the global stack and not just from the
  definition container because they can be overriden by other
  containers.

cura/Settings/ExtruderStack.py

@@ -115,6 +115,11 @@ class ExtruderStack(CuraContainerStack):
             if has_global_dependencies:
                 self.getNextStack().propertiesChanged.emit(key, properties)
 
+    def findDefaultVariant(self):
+        # The default variant is defined in the machine stack and/or definition, so use the machine stack to find
+        # the default variant.
+        return self.getNextStack().findDefaultVariant()
+
 
 extruder_stack_mime = MimeType(
     name = "application/x-cura-extruderstack",