Change az parameter to preferred_cache_cluster_azs

[odie5533]

https://github.com/cloudposse/terraform-aws-elasticache-redis/issues/173

what

The module should use preferred_cache_cluster_azs in the aws_elasticache_replication_group.

why

This would allow us to specify the preferred primary node. Also the parameter used for AZs, availability_zones, is deprecated.

references

https://github.com/cloudposse/terraform-aws-elasticache-redis/issues/173