Gusteau 1.0.1.dev with multi-node configuration

* Use the unified `.gusteau.yml` configuration file for all nodes and environments
* Support more advanced configuration (multiple nodes per environment)
* Provide 100% test coverage
* Use omnibus installation if platform is unspecified
* Update the template to include a test-kitchen setup with serverspec tests
* Implement `gusteau list` subcommand to list detected nodes from all environments