Return Redis endpoint data using unified format

This changes `redis_cluster_nodes_for_cfn_stack` to return endpoint
address and port as a hash, which is the same format used elsewhere.

This allows us to use the result of `redis_cluster_nodes_for_cfn_stack`
instead of `redis_cluster_replica_groups_for_cfn_stack` without changing
the consumer code at all. In other words: We can use the cluster node in
Puppet in case the replica group isn't available yet.