if outputdir is defined, write dates file there

This introduces a new variable OUTPUTDIR, which overrides the location where
the new dates file is written. This allow to run britney against a read-only
copy of the data.

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

britney.py

@@ -2368,6 +2368,9 @@ class Britney(object):
                 self.write_controlfiles(self.options.testing, 'testing')
 
             # write dates
+            try:
+                self.write_dates(self.options.outputdir, self.dates)
+            except AttributeError:
                 self.write_dates(self.options.testing, self.dates)
 
             # write HeidiResult