Rename .active-model.auto.conf

We don't want users to touch that file, so we use an extension
which is less likely to lead to that situation: we remove the
`.conf` extension.

References: BAR-122.

Signed-off-by: Israel Barth Rubio <[email protected]>

barman/config.py

@@ -765,7 +765,7 @@ def __init__(self, config, name):
                 value = None
             setattr(self, key, value)
         self._active_model_file = os.path.join(
-            self.backup_directory, ".active-model.auto.conf"
+            self.backup_directory, ".active-model.auto"
         )
         self.active_model = None
         self.models = {}