Add Cloud KMS support to GCS module (#87)

* Add support to Cloud KMS

* Fixes

* Fix tests

* Fix tests

* - change variable name to be consistent with BQ module
 - remove output, not needed
 - change string default value to null

* use locals to pre-populate kms key variable for all names

* rename kms variable, fix prefix check in locals

Co-authored-by: Ludovico Magnocavallo <ludomagno@google.com>

tests/modules/gcs/fixture/variables.tf

@@ -41,7 +41,7 @@ variable "labels" {
 
 variable "prefix" {
   type    = string
-  default = ""
+  default = null
 }
 
 variable "storage_class" {