Make spec.Prometheus.secret field optional

[pleshakov]

• Make the spec.Prometheus.secret optional so that Prometheus endpoint
  can be enabled without specifying a Secret.
• Extend the doc for the field.

If the field is not mark as optional, the admin will incorrectly see the following error:

kubectl apply -f ic.yaml
The NginxIngressController "my-nginx-ingress-controller" is invalid: spec.prometheus.secret: Required value

For ic.yaml:

apiVersion: k8s.nginx.org/v1alpha1
kind: NginxIngressController
metadata:
  name: my-nginx-ingress-controller
spec:
  type: deployment
  image:
    repository: nginx/nginx-ingress
    tag: 1.12.0
    pullPolicy: Always
  serviceType: NodePort
  prometheus:
    enable: true