disable app by app check

[MikeMichel]

the app by app config check marks all our apps invalid because we have a custom HAPROXY_HTTPS_FRONTEND_HEAD to capture all let's encrypt challenges

frontend marathon_https_in
  bind *:443 ssl {sslCerts}
  mode http
  acl letsencrypt_challenge path_beg /.well-known/acme-challenge
  http-request allow if letsencrypt_challenge
  use_backend locksmith if letsencrypt_challenge

the backend locksmith does not exist when the app by app check is done so the config is invalid for every checked app.

the backend itself is a dynamic container so we can not hard code it via template. a disable "app by app check" option for marathon-lb or ignoring referenced backends from templates would help us here.

[MikeMichel]

the patch from zliw is working. any chance to get it merged?