Limit projects to just one cluster

app/services/clusters/create_service.rb

@@ -5,6 +5,8 @@ module Clusters
     def execute(access_token)
       @access_token = access_token
 
+      return unless can_create_cluster?
+
       create_cluster.tap do |cluster|
         ClusterProvisionWorker.perform_async(cluster.id) if cluster.persisted?
       end
@@ -25,5 +27,13 @@ module Clusters
 
       @cluster_params = params.merge(user: current_user, projects: [project])
     end
+
+    def can_create_cluster?
+      if project.clusters.empty?
+        true
+      else
+        false
+      end
+    end
   end
 end