Add support for IngressClass resources

[lucacome]

Supporting IngressClass resource and ingressClassName for versions of k8s >= 1.18.

When using version of k8s >= 1.18 the use-ingress-class-only is now ignored and the Ingress Controller will only handle resources with the appropriate class set.

Fixes #924

[Dean-Coakley]

@lucacome This looks great!

Just one thing to make you aware of, is that you can set a compatible kubernetes version in your Chart.yaml. https://helm.sh/docs/topics/charts/#the-chartyaml-file

Example: kubeVersion: 1.15-1.19.

This would reject the installation of a helm release at the helm client level. I think this could be a nice improvement to include as it shortens the feedback loop for a user. (They don't have to install incompatible resources into their cluster to see logs that their cluster is incompatible.)

$ helm install test deployments/helm-chart
Error: chart requires kubeVersion: 1.15-1.19 which is incompatible with Kubernetes v1.14.0

$ helm ls
$

Feel free to ignore, we can always refer back to: https://docs.nginx.com/nginx-ingress-controller/technical-specifications/#supported-kubernetes-versions and hope most users read that.

[pleshakov]

Hi @lucacome

Please see my comments and suggestions. Most of them about docs. Looks good overall.

Additionally,

(1) We're changing behavior for VirtualServer/VirtualServerRoutes, right? Previously, if `"-use-ingress-class-only" was enabled, the IC would ignore VS/VSRs with empty class. Now it won't ignore them. If that's the case, could you add the change label to the PR? We will also need to document that change in the changelog.

(2) Could you include in the description that you're fixing this #924 ?

[Dean-Coakley]

LGTM pending #1133 (comment)

(Sorry if my example misled you @lucacome 😓 )

[pleshakov]

@lucacome looks good. Could you fix the mentioned link problems before merging?