Ship a Kubernetes JGroups stack (DNS_PING) and make the cluster name configurable

Running XWiki as several pods needs a JGroups stack that discovers members through the DNS records of a headless Service. dns.DNS_PING is part of JGroups core, but the WAR ships no such stack, so every deployment has to write and mount its own.

Proposal: bundle WEB-INF/observation/remote/jgroups/kubernetes.xml (TCP + DNS_PING, parameters through jgroups.* system properties) and add observation.remote.clusterName (default event, the value hard-coded today) so unrelated XWiki clusters on the same network stay separate.