fix interacting with insecure HTTPS registries

[aler9]

The Docker daemon allows to interact with insecure registries served through plain HTTP or served through HTTPS with self-signed certificates, when the target registry is included inside insecureRegistries. In this library it should be possible to interact with insecure registries likewise by using the name.Insecure option when creating references.

Nonetheless it's currently not possible to interact with insecure registries served with HTTPS and self-signed certificates, since the TLS certificate is checked anyway and an "invalid certificate" error is returned.

A common workaround consists into passing a tls.Config with InsecureSkipVerify set to true, but this disables TLS validation for every HTTP request, while the desired behavior is disabling TLS validation only when name.Insecure is in use.

This patch changes the default remote options in order to provide a default tls.Config with InsecureSkipVerify set to true if and only if name.Insecure is in use.

This also fixes bugs in dependent tools like Skaffold, that are passing name.Insecure and not InsecureSKipVerify as arguments to this library and are expecting to be able to interact with insecure HTTPS registries anyway.

Sample code:

ref, err := name.ParseReference("my.insecure.registry", name.Insecure)
if err != nil {
	panic(err)
}

img, err := remote.Image(ref, remote.WithAuth(&authn.Basic{Username: "user", Password: "pass"}))
if err != nil {
	panic(err)
}

fmt.Println(img)

I am expecting this to work when my.insecure.registry is either a plain HTTP server or a self-signed HTTPS server, but it works only in the former case.