Make container_vm the default image type

The default makes scripts more backwards compatible, since the default
image is still kinda broken. See issue #58 for discussion.

src/lib/command_line.ml

@@ -179,7 +179,9 @@ let main () =
           (i_need gke_name "A cluster-name is required for GKE clusters.")
           ~zone:(i_need gzone "A GCloud-zone name is required for GKE clusters.")
           ~max_nodes
-          ?image_type
+          ~image_type:(
+            Option.value_map image_type ~default:`Default ~f:(fun s -> `Set s)
+          )
           ?machine_type
         |> Cluster.gke
       | `Local_docker ->

src/lib/gke_cluster.ml

@@ -6,7 +6,7 @@ type t = {
   min_nodes: int [@default 1];
   max_nodes: int;
   machine_type: string [@default "n1-highmem-8"];
-  image_type: string option;
+  image_type: [ `Set of string | `Default] [@default (`Set "container_vm")];
 } [@@deriving yojson, show, make]
 
 let save ~storage:st cluster =
@@ -39,7 +39,9 @@ let max_started_jobs cluster =
 let gcloud_start ~log t =
   let cmd =
     let image_type_option =
-      Option.value_map t.image_type ~default:"" ~f:(sprintf "--image-type=%s")
+      match t.image_type with
+      | `Default -> ""
+      | `Set i -> sprintf "--image-type=%s" i
     in
     sprintf 
       "gcloud container clusters create %s %s \

src/lib/gke_cluster.mli

@@ -6,14 +6,14 @@ type t = private {
   min_nodes : int;
   max_nodes : int;
   machine_type : string;
-  image_type: string option;
+  image_type: [ `Set of string | `Default] [@default (`Set "container_vm")];
 } [@@deriving yojson, show]
 
 val make :
   zone:string ->
   ?min_nodes:int -> max_nodes:int ->
   ?machine_type: string ->
-  ?image_type: string ->
+  ?image_type: [ `Set of string | `Default] ->
   string -> t
 
 val max_started_jobs: t -> int