Control coredns hosts config map

[ytsarev]

• Collect IPs of healthy worker nodes
• Collect ingress hostnames with healthy services in the backend
• Update gslb.hosts data section of coredns config map
• Enhance health status resolution - rely on healthy EndpointSubset.Addresses
• Associated test suite extension/refactoring

[ytsarev]

Most recent spec/status example:

Spec:
  Ingress:
    Rules:
      Host:  app.cloud.absa.internal
      Http:
        Paths:
          Backend:
            Service Name:  app
            Service Port:  http
          Path:            /
      Host:                app1.cloud.absa.external
      Http:
        Paths:
          Backend:
            Service Name:  unhealthy-nginx
            Service Port:  http
          Path:            /
      Host:                app2.cloud.absa.external
      Http:
        Paths:
          Backend:
            Service Name:  healthy-nginx
            Service Port:  http
          Path:            /
  Strategy:                roundRobin
Status:
  Healthy Workers:
    Kind - Control - Plane:  172.17.0.2
  Managed Hosts:
    app.cloud.absa.internal
    app1.cloud.absa.external
    app2.cloud.absa.external
  Service Health:
    app.cloud.absa.internal:   NotFound
    app1.cloud.absa.external:  Unhealthy
    app2.cloud.absa.external:  Healthy

[ytsarev]

@donovanmuller While being distracted on status function fixes I forgot to provide core example for this PR :)

$ kubectl get svc gslb-coredns-coredns
NAME                   TYPE       CLUSTER-IP      EXTERNAL-IP   PORT(S)                     AGE
gslb-coredns-coredns   NodePort   10.98.255.208   <none>        53:30649/UDP,53:32411/TCP   4d17h

kubectl get ing
NAME           HOSTS                                                                       ADDRESS   PORTS   AGE
example-gslb   app.cloud.absa.internal,app1.cloud.absa.external,app2.cloud.absa.external             80      19h

$ kubectl run -it --rm --restart=Never --image=infoblox/dnstools:latest dnstools
If you don't see a command prompt, try pressing enter.

dnstools# dig +short @10.98.255.208 app.cloud.absa.internal
dnstools# dig +short @10.98.255.208 app1.cloud.absa.external
dnstools# dig +short @10.98.255.208 app2.cloud.absa.external
172.17.0.2

In the above example only healthy host/service is getting resolved by coredns