Allow to substitute environment variables in configuration

[ikatlinsky]

Feature request

Please describe your feature

We need a way to dynamically configure dashboard users based on the environment variables. The current implementation of config parsing using Viper does not support such an ability, all values are read as they are, without mapping to the values of the corresponding environment variables.

Describe the solution you'd like

Allow using environment variables in config.yaml in the following format

authentication:
      users:
        - username: $ADMIN_USERNAME_ENV_KEY
          password: $ADMIN_PASSWORD_ENV_KEY

This can be achieved by using Viper decoder hooks

Decoder:

func substituteEnvironmentVariables() mapstructure.DecodeHookFuncKind {
	return func(
		f reflect.Kind,
		t reflect.Kind,
		data interface{}) (interface{}, error) {
		if f != reflect.String || t != reflect.String {
			return data, nil
		}

		return os.ExpandEnv(data.(string)), nil
	}
}

Using a decoder in Viper

viper.Unmarshal(&config, viper.DecodeHook(substituteEnvironmentVariables()))