[terraform-ibm-account-infrastructure-base] Provide more flexibility on resource naming

[ocofaigh]

Currently in the DA solution we use prefix for all naming:

module "account_infrastructure_base" {
  source = "../.."
  providers = {
    logdna.at = logdna.at
    logdna.ld = logdna.ld
  }
  region                              = var.region
  resource_group_name                 = "${var.prefix}-resource-group"
  cos_instance_name                   = "${var.prefix}-cos-instance"
  cos_bucket_name                     = "${var.prefix}-cos-bucket"
  trusted_profile_name                = "${var.prefix}-trusted-profile"
  cos_target_name                     = "${var.prefix}-cos-target"
  cos_bucket_management_endpoint_type = var.cos_bucket_management_endpoint_type
  activity_tracker_route_name         = "${var.prefix}-cos-route"
  kms_key_crn                         = var.kms_key_crn
  resource_tags                       = var.resource_tags
}

We should create new optional variables which consumers can use to have full control over naming. If they are not used (null), fallback to using the prefix value