Fix constraints violation on is_highest in import

This field is not correct anyway and it is removed in newer versions. We
only set it to False here hoping it will not be worse but also as the
minimal possible change.

This is a backportable version of the fix.

relates to #1986

[noissue]

(cherry picked from commit 0474b11)

CHANGES/1986.bugfix

@@ -0,0 +1 @@
+Set `is_highest` to `False` before importing a collection version to prevent conflicts.

pulp_ansible/app/modelresource.py

@@ -115,6 +115,9 @@ def before_import_row(self, row, **kwargs):
 
         col = Collection.objects.get(name=row["name"], namespace=row["namespace"])
         row["collection"] = str(col.pk)
+        # This field easily produces constraints violations.
+        # But it's neither useful nor correct. It's removed in newer versions anyway.
+        row["is_highest"] = False
 
     def set_up_queryset(self):
         """