cloudxns: provider deprecation

[ldez]

Fixes #2323

[dmke]

May I propose a more radical approach where cloudxns will be reduced to something like

// keep consts, type Config, and type DNSProvider

var ErrDeadProvider = errors.New("cloudxns: provider has shut down")

func NewDNSProvider() (*DNSProvider, error) {
    return nil, ErrDeadProvider
}

func NewDNSProviderConfig(*Config) (*DNSProvider, error) {
    return nil, ErrDeadProvider
}

func (d *DNSProvider) Present(string, string, string) error {
    return ErrDeadProvider
}

func (d *DNSProvider) CleanUp(string, string, string) error {
    return ErrDeadProvider
}

When their website/domain is already offline, I don't see why we need to keep any code for it.

This way, users get an early and explicit reason ("provider has shut down" vs. "Get : context deadline exceeded")