Allow disabling of nginx-ingress

[loomsen]

What would you like to be added:

I'd like to be able to disable the default ingress installation, by setting ingress_nginx_enabled to false or something like that, similar to linkerd. There seems to be missing the switch for it here, it doesn't check for this var and add a count accordingly. https://github.com/run-x/opta/blob/20be2394e42703281c63a0794a23fa0dfd9b214e/modules/aws_k8s_base/tf_module/ingress_nginx.tf#L42

Why is this needed:

To be able to opt out if the ingress-nginx installation.

Extra info (e.g. existing slack convo link):

Slack thread