Support for advanced k8s resource overlays for configuration #302

Problem
The operator currently supports limited customization options via the crd (e.g. ingress selectors, registry credentials etc). For often-used customizations, It´s great if they are part of the crd. There are however many more customizations that sometimes require adjustment ( e.g. #160 #161 ). Adding all possibilities to the crd might not be an good idea.

The istio-operator supports so called "advanced k8s resource overlays" https://github.com/istio/operator#advanced-k8s-resource-overlays. To me, this looks like a great escape hatch to enable advanced customization of all values that are not yet (and probably will never be) exposed via the crd.