get images configurable from different registries

[ThormaehlenFred]

What would you like to be added:
Consume images from different configurable registries, like Iron Bank

Why is this needed:
Gardener consumers copy and adapt the original Gardener images and upload them to own public or private registries. Especially in the public sector area this is quite common. One example is Iron Bank. Iron Bank is a container registry maintained by the Platform One team. In order for a container image to be hosted in Iron Bank and marked approved, the image must pass a stringent container hardening. With this functionality a Gardener consumer can use garden-setup in combination with those individually hardened images residing in Iron Bank.