fix: add back the fix that was added in [v1.5.4](https://github.com/terraform-ibm-modules/terraform-ibm-event-notifications/releases/tag/v1.5.4) but incorrectly removed in [v1.6.0](https://github.com/terraform-ibm-modules/terraform-ibm-event-notifications/releases/tag/v1.6.0) (#227)

Co-authored-by: Steve Peggs <peggs@uk.ibm.com>

Author: shemau

solutions/standard/main.tf

@@ -78,7 +78,7 @@ module "kms" {
 locals {
   # tflint-ignore: terraform_unused_declarations
   validate_cos_regions        = var.cos_bucket_region != null && var.cross_region_location != null ? tobool("Cannot provide values for var.cos_bucket_region and var.cross_region_location") : true
-  cos_kms_key_crn             = var.existing_cos_bucket_name != null ? null : local.existing_kms_root_key_id != null ? local.existing_kms_root_key_id : module.kms[0].keys[format("%s.%s", var.cos_key_ring_name, var.cos_key_name)].crn
+  cos_kms_key_crn             = var.existing_cos_bucket_name != null ? null : var.existing_kms_root_key_crn != null ? var.existing_kms_root_key_crn : module.kms[0].keys[format("%s.%s", var.cos_key_ring_name, var.cos_key_name)].crn
   cos_instance_guid           = var.existing_cos_instance_crn != null ? element(split(":", var.existing_cos_instance_crn), length(split(":", var.existing_cos_instance_crn)) - 3) : module.cos.cos_instance_guid
   cos_bucket_name             = var.existing_cos_bucket_name != null ? var.existing_cos_bucket_name : (var.prefix != null ? "${var.prefix}-${var.cos_bucket_name}" : var.cos_bucket_name)
   cos_bucket_name_with_suffix = var.existing_cos_bucket_name != null ? var.existing_cos_bucket_name : module.cos.bucket_name