Make adt_baseline config optional

Given that only one value is defined ("reference"), it is a better
option to allow the config to be unset when one does not want to have
adt_baseline set to "reference".

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

britney2/policies/autopkgtest.py

@@ -108,6 +108,10 @@ class AutopkgtestPolicy(BasePolicy):
         os.makedirs(self.options.state_dir, exist_ok=True)
         self.read_pending_tests()
 
+        if not hasattr(self.options, 'adt_baseline'):
+            # Make adt_baseline optional
+            setattr(self.options, 'adt_baseline', None)
+
         # read the cached results that we collected so far
         if os.path.exists(self.results_cache_file):
             with open(self.results_cache_file) as f: