Clarify variable usage with descriptions

shared-variables.tf

@@ -1,6 +1,26 @@
-variable "environment" {}
-variable "ami" {}
-variable "account" {}
-variable "region" {}
-variable "enable_guacamole" {}
-variable "enable_inetsim" {}
+variable "environment" {
+  description = "The name of the environment in which the resources will be created'"
+  default     = "malware-lab"
+}
+variable "ami" {
+  description = "The Amazon Machine Image (AMI) ID of the created FlareVM."
+}
+
+variable "account" {
+  description = "The AWS account ID to use for the resources"
+}
+
+variable "region" {
+  description = "The AWS region to use for the resources"
+  default     = "eu-west-1"
+}
+
+variable "enable_guacamole" {
+  description = "Whether to enable the Guacamole server for remote access to the instances (If enabled the FlareVM will have not Internet)"
+  default     = false
+}
+
+variable "enable_inetsim" {
+  description = "Whether to enable the InetSim network simulation tool on the instances"
+  default     = false
+}