Adds IAM roles for the Managed Identity Module

This adds the ability to pass in a list of roles to be assigned to the
managed identity user.

terraform/modules/managed_identity/variables.tf

@@ -22,3 +22,8 @@ variable "identity" {
   type        = string
   description = "Name of the managed identity to create"
 }
+
+variable "roles" {
+  type        = list
+  description = "List of roles by name"
+}