Adding install_snippets support.

During the "real" first boot (install boot), we need tu run butane
config to manipulate disks, so we add install_snippets variable to do
so.

This snippets are added to the install.yaml butane configuration

bare-metal/flatcar-linux/kubernetes/worker/matchbox.tf

@@ -55,6 +55,7 @@ data "ct_config" "install" {
     baseurl_flag = var.cached_install ? "-b ${var.matchbox_http_endpoint}/assets/flatcar" : ""
   })
   strict = true
+  snippets = var.install_snippets
 }
 
 # Match a worker to a profile by MAC

bare-metal/flatcar-linux/kubernetes/worker/variables.tf

@@ -60,6 +60,12 @@ variable "snippets" {
   default     = []
 }
 
+variable "install_snippets" {
+  type        = list(string)
+  description = "List of Butane snippets to run with the install command"
+  default     = []
+}
+
 variable "node_labels" {
   type        = list(string)
   description = "List of initial node labels"