Remove list of applications from update_service and create_service scope

As commented here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25386#note_147766954

Could we DRY-up our code a bit and also remove the responsibility of knowing the updatable and creatable apps from app/services/clusters/applications/update_service.rb and app/services/clusters/applications/create_service.rb?