v2/images: remove source and add URL parameter

[tormath1]

The source option was tied to the FTP based workflow which is no longer supported. Thus the option would trigger an unresolvable error for the SDK user.

(From: brightbox/brightbox-cli@10cb209)

---

Locally tested with:

img, err := client.CreateImage(ctx, brightbox.ImageOptions{
	Arch:              arch.X86_64,
	Name:              &name,
	Username:          &username,
	MinRAM:            &ram,
	URL:               "https://...flatcar_production_openstack_image.img",
	Public:            &public,
	CompatibilityMode: &compatibilityMode,
})
if err != nil {
	log.Fatal(err)
}

fmt.Println(img)

[NeilW]

Thanks. Tagged that as v2.2.0