Simplify the local configuration file syntax

Here is the current local configuration file format:

[DEFAULT]
env = custom

[custom]
log-level-file = DEBUG

It is practical to quickly switch between "env", mais in reality nobody is using it and several users are complaining that their config file is not taken into account. When it happens, most of the time their config file looks like:

log-level-file = DEBUG

Which is absolutely correct. But it is not what Drive understands.

So we need to simplify the syntax of the config file: it must accept simple options, one by line, without the need for the "DEFAULT" section nor the "env" one.

It is important to keep the compatibility with the current format. We also do not want to alter the current config file format of our users as it will break if they downgrade their version of the application.