Allow G5K API login through environment variables instead of python-grid5000.yaml

We are using Enoslib as an "executor" library for a bigger project, and having to put G5K credentials in ~/.python-grid5000.yaml is not very convenient when deploying our code. It also poses a security risk because the password is stored in cleartext.

We would like to pass the G5K credentials through environment variables instead. These variables would then be set by Docker.