Use strict mode for Container Linux Configs

* Enable terraform-provider-ct `strict` mode for parsing
Container Linux Configs and snippets
* Fix Container Linux Config systemd unit syntax `enable`
(old) to `enabled`
* Align with Fedora CoreOS which uses strict mode already

google-cloud/fedora-coreos/kubernetes/versions.tf

@@ -4,7 +4,7 @@ terraform {
   required_version = "~> 0.12.6"
   required_providers {
     google   = ">= 2.19, < 4.0"
-    ct       = "~> 0.3"
+    ct       = "~> 0.4"
     template = "~> 2.1"
     null     = "~> 2.1"
   }