Add environment variables to wsgi script
As requested on [1] it will be a good addition to allow to configure some of the wsgi application behaviour by using environemnt variables. Although it's possible to use a custom wsgi script, I think it's a good practice to provide a configurable default one.
We should include the following variables:
* TRYTOND_LOGGING_CONFIG: Which will be the route to the file used as logging configuration
* TRYTOND_DATABASE_NAME: The database name to initialize when initialitzing the server. This will be required when implementing #5694.
[1] http://www.tryton.org/~irclog/2017-03-06.log.html#t2017-03-06%2017:38