Return endpoint data using unified format

This changes

* `redis_cluster_nodes_for_cfn_stack and`
* `memcached_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.