added create_read_replica support for elasticache (redis)

Essentially the same as https://github.com/boto/boto/pull/1711

> Amazon's documentation is incomplete in this regard, but after playing with the web interface and the API I figured out how it works.

> If you launch a redis cache cluster with a replication_group_id, that cluster becomes a read-only replica cluster in the replication group. BUT if you do that, then you can't send up any other parameters, including the "required" ones (engine, num_nodes, and cache_node_type). The one exception I have found to that is the preferred_availability_zone parameter, which I was able to figure out only because it's an option on their web interface.

> Since Amazon is essentially using the same endpoint for two logically different functions, I split the create_cache_cluster command into create_cache_cluster and create_replica_cluster to match. You could fix the behavior by not making engine, num_nodes, and cache_node_type required arguments, but I think that is significantly less clear.