Make UPGRADE_OUTPUT optional

All the output is a duplicate of what is being sent to the root logger
(albeit in a different format) and in e.g. our test suite the output
is entirely redundant.

Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -2824,11 +2824,16 @@ class Britney(object):
             self.upgrade_me = self.options.actions.split()
 
         if self.options.compute_migrations or self.options.hint_tester:
-            file_handler = logging.FileHandler(self.options.upgrade_output, mode='w', encoding='utf-8')
+            if hasattr(self.options, 'upgrade_output'):
+                upgrade_output = getattr(self.options, 'upgrade_output')
+                file_handler = logging.FileHandler(upgrade_output, mode='w', encoding='utf-8')
                 output_formatter = logging.Formatter('%(message)s')
                 file_handler.setFormatter(output_formatter)
                 self.output_logger.addHandler(file_handler)
-            self.logger.info("Logging upgrade output to %s", self.options.upgrade_output)
+                self.logger.info("Logging upgrade output to %s", upgrade_output)
+            else:
+                self.logger.info("Upgrade output not (also) written to a separate file"
+                                 " as the UPGRADE_OUTPUT configuration is not provided.")
 
             # run the hint tester
             if self.options.hint_tester: