Fixes #4872 - Add exclusion for ignore_interfaces.st if already existing during rudder-upgrade

Fixes #4872 - Add exclusion for ignore_interfaces.st if already existing during rudder-upgrade

cf http://www.rudder-project.org/redmine/issues/4872
I don't understand the logic. You can exclude a file even if it doesn't exist. 
Wouldn't it be simpler ?