Allow to set configuration value from environment
It is difficult to configure the docker image without:
* having to create a new image which need to be kept up to date
* mount a volume to override the configuration file which need a local file
So I propose to allow to set default value for any configuration option using environment variables.
I propose this syntax: TRYTOND_
This allow to have '_' in section name (which we already have).
Also all the sections and option names in Tryton configuration must be in lower case (which is already the case).