Allow deployment type Istio in the Product CRD

It seems that only `spec.deployment.apicastHosted` and `spec.deployment.apicastSelfManaged` are expected in the Product CRD. I made some tests here trying `apicastIstio` and `istio` and I'm getting error:

Error "Invalid value: "": "spec.deployment" must validate one and only one schema (oneOf). Found none valid" for field "<nil>". 

The purpose of this RFE is to consider `spec.deployment.apicastIstio` in the Product CRD. .e.g.

 apiVersion: capabilities.3scale.net/v1beta1
kind: Product
metadata:
  name: product1
spec:
  name: "OperatedProduct 1"
  deployment:
    apicastIstio: {}