Use auto-issued TLS certificate for inter-node communication

[monrax]

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