Cherry-pick of PR #16197 to 7.x branch. Original message:

Resolves: #8451

This PR adds fileset for ingress-nginx controller logs. Log patterns could be found on the controllers' docs.

In order to add the new pipeline a new setting called "version" is being introduced also in this PR. See the sum-up of the discussion on this comment.

How to test this

Filebeat

0. in /tmp/testnginx one can use the sample logs(see test.log file) used for automated testing.
1. Enable nginx Filebeat module
2. Configure the fileset.

- module: nginx
  # Access logs
  ingress_controller:
    enabled: true
    var.paths: ["/tmp/testnginx"]

3. Setup the pipelines in ES: ./filebeat setup
4. Start Filebeat and verify that events are properly shown in Kibana. NOTE: set the time window properly in Kibana so as to display the events in the timeline since the events have timestamps which will be in the past.

k8s environment

1. Deploy an ingress managed sample app on Minikube following the official k8s docs
2. check that ingress controller logs are being populated when doing curls (like described in k8s docs). Ingress controller pod lies under kube-system namespace and you will be able to check its logs with sth like nginx-ingress-controller-6fc5bcc8c9-zm8zv.
3. Redirect pods' logs to a temporary file: kubectl -n kube-system logs -f nginx-ingress-controller-6fc5bcc8c9-zm8zv >> /tmp/ingresspod
4. Inside minikube configure nginx module like:

- module: nginx
  # Ingress-nginx controller logs. This is disabled by default. It could be used in Kubernetes environments to parse ingress-nginx logs
  ingress_controller:
    enabled: true

    # Set custom paths for the log files. If left empty,
    # Filebeat will choose the paths depending on your OS.
    var.paths: ["/tmp/ingresspod"]

5. Run filebeat and check that logs are parsed correctly and shown in Kibana properly.
6. Visit http://hello-world.info/v2 so as to create new log entries.