[database][models] Fixed get_by_natural_key of DatabaseSetOutputManager

The natural key generate was using a tuple in a tuple (which is good)
however the get_by_natural_key method was expecting a flat tuple.

beat/web/databases/models.py

@@ -437,15 +437,10 @@ class DatabaseSetTemplateOutput(models.Model):
 
 
 class DatabaseSetOutputManager(models.Manager):
-
-    def get_by_natural_key(self, database_name, database_version, protocol_name, name,
-                           template_name, output_name):
+    def get_by_natural_key(self, set_natural_key, output_name):
+        set_ = DatabaseSet.objects.get_by_natural_key(*set_natural_key)
         return self.get(
-            set__protocol__database__name=database_name,
-            set__protocol__database__version=database_version,
-            set__protocol__name=protocol_name,
-            set__name=name,
-            set__template__name=template_name,
+            set=set_,
             template__name=output_name,
         )