Allow to pass many configuration file to trytond
The idea is to be able to share some part of the configuration between many instances while keeping other parts specific to each instance.
So the config parser will parse sequentially each files like the ConfigParser does [1].
The "--config" argument will become a nargs="+".
[1] https://docs.python.org/2/library/configparser.html#ConfigParser.RawConfigParser.read