Move db_name default into config_spec.ini

The default name of the database ("mediagoblin") was coded
in db/open.py and init/celery/__init__.py.
Instead use the new config system to have this as the
default for the "db_name" config option.

mediagoblin/db/open.py

@@ -30,7 +30,7 @@ def connect_database_from_config(app_config):
 
 def setup_connection_and_db_from_config(app_config):
     connection = connect_database_from_config(app_config)
-    database_path = app_config.get('db_name', 'mediagoblin')
+    database_path = app_config['db_name']
     db = connection[database_path]
     models.register_models(connection)
     # Could configure indexes here on db