Use auto-issued TLS certificate for inter-node communication

A managed `Certificate` resource from `cert-manager.io/v1` could be deployed (similar to the managed `Issuer`) where both a `commonName` and `dnsNames` could be specified to be used not to terminate TLS connectons at the Ingress level, but also to secure connections between Graylog nodes, as long as another certificate isn't provided.

For example:

https://github.com/mongodb/mongodb-kubernetes/blob/620a9a3ce9a3675f2b4c99663611cb3452015375/helm_chart/templates/mongodbcommunity_cr_with_tls.yaml#L38-L52