Added environments support for chef-solo

In response to https://github.com/opscode/test-kitchen/issues/199, I have added environment support for `chef-solo`.

To configure this support, the `.kitchen.yml` file will look like:
```
suites:
  - name: default
    encrypted_data_bag_secret_key_path: "encrypted_data_bag_secret"
    data_bags_path: "../../data_bags"
    roles_path: "../../roles"
    environments_path: "../../environments"
    environment: "dev"
    run_list: ["recipe[foo]", "recipe[minitest-handler]"]
```

If no `environment` is defined, the `_default` environment will still be applied when `chef-solo` runs.