Refactor cluster app creation code in controller

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/controllers/projects/clusters/applications_controller.rb

@@ -5,17 +5,18 @@ class Projects::Clusters::ApplicationsController < Projects::ApplicationControll
   before_action :authorize_create_cluster!, only: [:create]
 
   def create
-    application = @application_class.find_or_create_by!(cluster: @cluster)
+    application = @application_class.find_or_initialize_by(cluster: @cluster)
 
-    if application.respond_to?(:hostname)
-      application.update(hostname: params[:hostname])
+    if has_attribute?(:hostname)
+      application.hostname = params[:hostname]
     end
 
     if application.respond_to?(:oauth_application)
       application.oauth_application = create_oauth_application(application)
-      application.save
     end
 
+    application.save!
+
     Clusters::Applications::ScheduleInstallationService.new(project, current_user).execute(application)
 
     head :no_content