fixed bastion subnet to list

bastion/variables.tf

@@ -2,6 +2,7 @@ variable "vpc_id" {
 }
 
 variable "subnet_public" {
+  default     = []
 }
 
 variable "allowed_network" {
@@ -50,4 +51,3 @@ variable "centos7_amis" {
         sa-east-1 = "ami-bf9520a2"
     }
 }
-