Service discovery and exposition

[siegfriedweber]

Let the user choose the exposed role-groups and provide a discovery ConfigMap

Part of #1

Tasks

• Decision: Service discovery and exposition #54
• Rename the current cluster-manager discovery service, assign the TLS scope only to the role-groups with the cluster-manager role and change discovery.seed_hosts if setting this service instead of the individual pods is not correct. Check also cluster.initial_cluster_manager_nodes.
• Implement