registry: Add subscription tier slug argument to create

As of GA, users are required to specify a subscription tier slug when creating a
registry. The subscription is also returned in the resposne, but for now we
ignore it to keep API compatibility.

registry.go

@@ -45,7 +45,8 @@ type RegistryServiceOp struct {
 
 // RegistryCreateRequest represents a request to create a registry.
 type RegistryCreateRequest struct {
-	Name string `json:"name,omitempty"`
+	Name                 string `json:"name,omitempty"`
+	SubscriptionTierSlug string `json:"subscription_tier_slug,omitempty"`
 }
 
 // RegistryDockerCredentialsRequest represents a request to retrieve docker

registry_test.go

@@ -51,15 +51,29 @@ func TestRegistry_Create(t *testing.T) {
 	}
 
 	createRequest := &RegistryCreateRequest{
-		Name: want.Name,
+		Name:                 want.Name,
+		SubscriptionTierSlug: "basic",
 	}
 
 	createResponseJSON := `
 {
 	"registry": {
 		"name": "` + testRegistry + `",
         "created_at": "` + testTimeString + `"
-	}
+	},
+    "subscription": {
+      "tier": {
+        "name": "Basic",
+        "slug": "basic",
+        "included_repositories": 5,
+        "included_storage_bytes": 5368709120,
+        "allow_storage_overage": true,
+        "included_bandwidth_bytes": 5368709120,
+        "monthly_price_in_cents": 500
+      },
+      "created_at": "` + testTimeString + `",
+      "updated_at": "` + testTimeString + `"
+    }
 }`
 
 	mux.HandleFunc("/v2/registry", func(w http.ResponseWriter, r *http.Request) {