added vars for clusternet/sub, dmznet/sub, and borderrouter

vars.tf

+variable "internal-net" {
+  default = "clusternet"
+}
+
+variable "internal-subnet" {
+  default = "clustersubnet"
+}
+
+variable "external-net" {
+  default = "dmznet"
+}
+
+variable "external-subnet" {
+  default = "dmzsubnet"
+}
+variable "router" {
+  default = "borderrouter"
+
 variable "image" {
   default = "CentOS-7-x86_64-GenericCloud-1905"
 }