Expose a few more configurable items and add input validators

variable.host_name.tf

@@ -1,6 +1,9 @@
 variable "host_name" {
   description = "The hostname for the droplet"
-  default     = "hello-world"
   type        = string
-}
 
+  validation {
+    condition     = can(regex("^[a-zA-Z0-9-]+$", var.host_name))
+    error_message = "The host name can only contain alphanumeric characters and hyphens."
+  }
+}