Make Heidi optional

Setting HEIDI_OUTPUT to an empty value will skip the heidi generation, to speed
up britney in scenarios where heidi is not needed.

britney.py

@@ -484,7 +484,7 @@ class Britney(object):
         if self.options.series == '':
             self.options.series = self.suite_info.target_suite.name
 
-        if not hasattr(self.options, "heidi_delta_output"):
+        if hasattr(self.options, 'heidi_output') and not hasattr(self.options, "heidi_delta_output"):
             self.options.heidi_delta_output = self.options.heidi_output + "Delta"
 
         self.options.smooth_updates = self.options.smooth_updates.split()
@@ -1273,6 +1273,7 @@ class Britney(object):
         if not self.options.dry_run:
             target_suite = self.suite_info.target_suite
 
+            if hasattr(self.options, 'heidi_output'):
                 # write HeidiResult
                 self.logger.info("Writing Heidi results to %s", self.options.heidi_output)
                 write_heidi(self.options.heidi_output,