Change requirement for OpenSearch dedicated master instance type

This changes the dedicated master so it is optional and not created by
default. It also makes the dedicated master instance type variable
optional, as we won't necessarily have it enabled.

opensearch-variables.tf

@@ -13,12 +13,13 @@ variable "es_instance_count" {
 variable "es_dedicated_master_type" {
   type        = string
   description = "Instance type of the dedicated main nodes in the cluster."
+  default     = ""
 }
 
 variable "es_instance_type" {
   type        = string
   description = "Instance type of data nodes in the cluster."
-  default     = ""
+  default     = "t3.small.search"
 }
 
 variable "es_volume_type" {