Add uuid for tracim instance in config

tracim/development.ini.base

 # Fallback language if browser and tracim can't find one they agree on.
 i18n.lang = en
 
+tracim_instance.uuid = 79160afe-e324-473a-9aef-7b93f69c1c1b
+
 cache_dir = %(here)s/data
 # preview generator cache directory
 preview_cache_dir = /tmp/tracim/preview/

tracim/tracim/config/app_cfg.py

         """Parse configuration file."""
         mandatory_msg = \
             'ERROR: {} configuration is mandatory. Set it before continuing.'
+        self.TRACIM_INSTANCE_UUID = tg.config.get(
+            'tracim_instance.uuid',
+        )
         self.DEPOT_STORAGE_DIR = tg.config.get(
             'depot_storage_dir',
         )