Prevent update configuration after modules are imported

Modules can store copies of the configuration for performance reason so we must
prevent to change the configuration after it happens.

issue9679
review318351002

proteus/config.py

@@ -232,7 +232,8 @@ class TrytondConfig(Config):
         if not config_file:
             config_file = os.environ.get('TRYTOND_CONFIG')
         from trytond.config import config
-        config.update_etc(config_file)
+        if config_file:
+            config.update_etc(config_file)
         from trytond.pool import Pool
         from trytond.transaction import Transaction
         self.database = database