added documentation

ohpc-instance/main.tf

 variable "ohpc_instance_name" {}
 variable "image_ohpc" {}
 variable "flavor" {}
+
+# is created in key-pair module and called in root module
 variable "key_pair" {type = "string"}
+
 variable "internal_net" {}
 variable "external_net" {}
+
+# is created in floating-ip module and called in root module
 variable "floating_ip_ohpc" {type = string}
+
 variable "host_prefix" {}
 variable "ohpc_user" {}
 variable "ssh_private_key" {}
+
+# is created in external-network module and called in root module
 variable "external_subnet_id" {type = "string"}