Support externalTrafficPolicy in the Logstash Helm chart

[kostasb]

Describe the feature:

Setting the externalTrafficPolicy to Local does not appear to have any effect in the resulting service.

  service:
    type: LoadBalancer
    externalTrafficPolicy: Local

kubectl describe svc logstash-logstash

...(omitted)
External Traffic Policy:  Cluster
...

Describe a specific use case for the feature:
There is a number of reasons why someone would need to set the externalTrafficPolicy to Local, namely:

There are two available options: Cluster (default) and Local. Cluster obscures the client source IP and may cause a second hop to another node, but should have good overall load-spreading. Local preserves the client source IP and avoids a second hop for LoadBalancer and NodePort type Services, but risks potentially imbalanced traffic spreading.

from https://kubernetes.io/docs/tasks/access-application-cluster/create-external-load-balancer/#preserving-the-client-source-ip