Support disable service retry

[cnef]

I need to disable retry for service, but the proxy set retries to 0, it doesn't apply to kong and the DB doesn't update.

apiVersion: configuration.konghq.com/v1
kind: KongIngress
metadata:
  name: service1
proxy:
  path: /
  protocol: http
  retries: 0
route:
  preserve_host: true

[hbagdi]

The default value for kongIngress.Proxy.Retries will be 0, meaning that this branch will evaluate to true if there a KongIngress resource associated with an Ingress.
This will change the behavior as it will set the retries to 0.

The reason is, we can't differentiate between an explicit user setting it to 0 and an absent value.
This can be solved by using pointers instead of plain int.

[cnef]

Thanks, It's updated, please review again.

[hbagdi]

@zhxcai You will need to update the auto generated code using ./hack/update-codegen.sh.

[hbagdi]

@zhxcai I merged this in manually with b8bb7bd. It'll be available in the next release.

Thank you for your contribution!