use a logging configuration file
According to Cédric [1], trytond should fail when logfile can't be opened [2].
[1] http://codereview.tryton.org/831002/diff/1/trytond/server.py#inline-preview-0-46-b
[2] http://hg.tryton.org/trytond/file/62f1b9747e9f/trytond/server.py#l43