Don't insert previously negated items Applications.append_if_new()

We add a check in `Applications.append_if_new()` to only insert
non-negated items if they aren't already on our internal negation list.

This fixes the issue where it wasn't possible to preemptively remove an
entry from the applications list in a multi-dimensional hierarchy, e.g.
in a [Commodore] global defaults repository where we may want to exclude
applications for a certain Kubernetes distribution regardless of the
cloud on which a cluster with that distribution is running.

[Commodore]: https://syn.tools/commodore

reclass/datatypes/applications.py

@@ -42,7 +42,9 @@ def append_if_new(self, item):
                 self._items.remove(item)
             except ValueError:
                 pass
-        else:
+        elif item not in self._negations:
+            # Only insert non-negated items if they're not in our negations list
+            # already.
             super(Applications, self)._append_if_new(item)
 
     def merge_unique(self, iterable):