Custom Unmarshaler should not accept viper

[bogdandrutu]

Currently the custom unmarshaler accepts viper:

// ConfigUnmarshaler interface is an optional interface that if implemented by a Factory,
// the configuration loading system will use to unmarshal the config.
type ConfigUnmarshaler interface {
	// Unmarshal is a function that un-marshals a viper data into a config struct in a custom way.
	// componentViperSection *viper.Viper
	//   The config for this specific component. May be nil or empty if no config available.
	// intoCfg interface{}
	//   An empty interface wrapping a pointer to the config struct to unmarshal into.
	Unmarshal(componentViperSection *viper.Viper, intoCfg interface{}) error
}

It would be good to not expose viper directly but an interface{} or map[string]interface{} in our public API, so if viper breaks compatibility or moves to viper v2 (which is already planned) we can upgrade without breaking compatibility.

[mx-psi]

I would be happy to work on a PR for fixing this and #2597 next week. Could I be assigned this?

[bogdandrutu]

This is done.