[neutron] Zuul checks fail

Hi neutrinos, a quick heads-up that currently Zuul will 100% give -1 on your reviews, failing 404 Not Found when trying to download Ubuntu Xenial image. Thanks to Yulong for filling launchpad bug [0] and submitting fix [1]. Once it is merged, we should be back in business [0] https://bugs.launchpad.net/neutron/+bug/1832968 [1] https://review.opendev.org/#/c/665530/ -- Bernard Cafarelli