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

@@ -471,7 +471,7 @@ class Britney(object):
             self.log("Cannot use --control-files with mirror-layout (components)!", type="E")
             sys.exit(1)
 
-        if not hasattr(self.options, "heidi_delta_output"):
+        if self.options.heidi_output and not hasattr(self.options, "heidi_delta_output"):
             self.options.heidi_delta_output = self.options.heidi_output + "Delta"
 
         self.options.nobreakall_arches = self.options.nobreakall_arches.split()
@@ -2478,6 +2478,7 @@ class Britney(object):
                 write_controlfiles(self.sources, self.binaries,
                                    'testing', self.suite_info['testing'].path)
 
+            if self.options.heidi_output:
                 # write HeidiResult
                 self.log("Writing Heidi results to %s" % self.options.heidi_output)
                 write_heidi(self.options.heidi_output, self.sources["testing"],