Support configuration from ENV

As mentioned in [two](https://github.com/triAGENS/guacamole/issues/47) [issues](https://github.com/triAGENS/guacamole/issues/63) we currently don't support configuration other than a static YAML file. This is very limiting especially for non-development environments since you don't want to check credentials into the repository. Recently the usage of tools like [dotenv](https://github.com/bkeepers/dotenv) emerged to solve these issues. Additionally Heroku made the [usage of connection URIs](https://devcenter.heroku.com/articles/heroku-postgresql#establish-primary-db) very popular.

Thus is it crucial for us to support these options. And here you go.
@moonglum Ashikawa don't accept username/password in the URI, thus must remove it from the URI. Setting user to nil will remove both.