Fix creation of Redis in cluster mode (#65)

* No number_cache_clusters if cluster_mode_enabled

See https://github.com/cloudposse/terraform-aws-elasticache-redis/issues/64

* No explicit AZs in cluster mode

* Updated README.md
Co-authored-by: actions-bot <58130806+actions-bot@users.noreply.github.com>

README.md

@@ -256,6 +256,10 @@ We deliver 10x the value for a fraction of the cost of a full-time engineer. Our
 
 Join our [Open Source Community][slack] on Slack. It's **FREE** for everyone! Our "SweetOps" community is where you get to talk with others who share a similar vision for how to rollout and manage infrastructure. This is the best place to talk shop, ask questions, solicit feedback, and work together as a community to build totally *sweet* infrastructure.
 
+## Discourse Forums
+
+Participate in our [Discourse Forums][discourse]. Here you'll find answers to commonly asked questions. Most questions will be related to the enormous number of projects we support on our GitHub. Come here to collaborate on answers, find solutions, and get ideas about the products and services we value. It only takes a minute to get started! Just sign in with SSO using your GitHub account.
+
 ## Newsletter
 
 Sign up for [our newsletter][newsletter] that covers everything on our technology radar.  Receive updates on what we're up to on GitHub as well as awesome new projects we discover. 
@@ -375,6 +379,7 @@ Check out [our other projects][github], [follow us on twitter][twitter], [apply
   [testimonial]: https://cpco.io/leave-testimonial?utm_source=github&utm_medium=readme&utm_campaign=cloudposse/terraform-aws-elasticache-redis&utm_content=testimonial
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   [newsletter]: https://cpco.io/newsletter?utm_source=github&utm_medium=readme&utm_campaign=cloudposse/terraform-aws-elasticache-redis&utm_content=newsletter
+  [discourse]: https://ask.sweetops.com/?utm_source=github&utm_medium=readme&utm_campaign=cloudposse/terraform-aws-elasticache-redis&utm_content=discourse
   [email]: https://cpco.io/email?utm_source=github&utm_medium=readme&utm_campaign=cloudposse/terraform-aws-elasticache-redis&utm_content=email
   [commercial_support]: https://cpco.io/commercial-support?utm_source=github&utm_medium=readme&utm_campaign=cloudposse/terraform-aws-elasticache-redis&utm_content=commercial_support
   [we_love_open_source]: https://cpco.io/we-love-open-source?utm_source=github&utm_medium=readme&utm_campaign=cloudposse/terraform-aws-elasticache-redis&utm_content=we_love_open_source

main.tf

@@ -85,10 +85,10 @@ resource "aws_elasticache_replication_group" "default" {
   replication_group_id          = var.replication_group_id == "" ? module.label.id : var.replication_group_id
   replication_group_description = module.label.id
   node_type                     = var.instance_type
-  number_cache_clusters         = var.cluster_mode_enabled ? (1 + var.cluster_mode_replicas_per_node_group) * var.cluster_mode_num_node_groups : var.cluster_size
+  number_cache_clusters         = var.cluster_mode_enabled ? null : var.cluster_size
   port                          = var.port
   parameter_group_name          = join("", aws_elasticache_parameter_group.default.*.name)
-  availability_zones            = slice(var.availability_zones, 0, var.cluster_size)
+  availability_zones            = var.cluster_mode_enabled ? null : slice(var.availability_zones, 0, var.cluster_size)
   automatic_failover_enabled    = var.automatic_failover_enabled
   subnet_group_name             = local.elasticache_subnet_group_name
   security_group_ids            = var.use_existing_security_groups ? var.existing_security_groups : [join("", aws_security_group.default.*.id)]