Allow configuration using environment variables

[hendrikhofstadt]

The Docker image should allow users to specify all of the config variables using environment variables.

Like e.g. LISK_FORGING_SECRET_<X> please consult #85 as a reference implementation.

Baking in a config.json file is not a best-practice and should not be done in order to avoid leaking secrets.

This will also make #78 much easier since ports, seed nodes etc. could be changed from e.g. a docker-compose file.