Webhook: Use cert-manager for certificate lifecycle management.

[Gacko]

Tests on workload clusters (not always required)

For changes in the chart, chart templates, and ingress controller container images, I executed the following tests
to verify them working in live enviromnents:

Test / Provider	AWS	Azure	KVM
Upgrade from previous version	✔
Existing Ingress resources are reconciled correctly	✔
Fresh install	✔
Fresh Ingress resources are reconciled correctly	✔

Testing was done using hello-world-app.

Hint for KVM:

kubectl port-forward -n kube-system svc/nginx-ingress-controller-app 8080:80
ingress_domain=host.configured.in.ingress; curl --connect-to "$ingress_domain:80:127.0.0.1:8080" "http://$ingress_domain" -v

[Gacko]

Note: This is an alignment to upstream, don't blame me for obvious hacks like hard-coded values. Guess I'll propose improvements later.