Exposing TRUFFLE_NETWORK in the node process

[lgalabru]

Hello there,

I'm proposing a quick work around, so we can work around the current lack of flexibility with the environment / deployment issue. The idea behind this patch is to expose the selected network as an environment variable, so that the process can introspect itself and be aware of the current environment.
This is a practice coming from the rails community. The command:
$ rails c production
will seed RAILS_ENV with production.
Thank you for the amazing work!
Best,

Ludovic