Comment 3 for bug 1867807

fix there

        * add backup tempest cirros url
        * Change-Id: Ibe5ff806539479fb94d4a61aff9ac06a883a6895
        * https://review.opendev.org/#/c/713505/3

bug also seen in cloudops scen2 today

        * https://logserver.rdoproject.org/openstack-component-cloudops/opendev.org/openstack/tripleo-ci/master/periodic-tripleo-ci-centos-8-scenario002-standalone-cloudops-master/21e4ed0/job-output.txt
        * 2020-03-18 06:20:11.467767 | primary | 2020-03-18 06:11:08.176 229109 INFO config_tempest.constants [-] Downloading 'https://download.cirros-cloud.net/0.4.0/cirros-0.4.0-x86_64-disk.img' and saving as 'etc/cirros-0.4.0-x86_64-disk.img'
2020-03-18 06:20:11.467778 | primary | 2020-03-18 06:13:17.477 229109 WARNING config_tempest.constants [-] <urlopen error [Errno 101] Network is unreachable>, Retrying in 3 seconds.: urllib.error.URLError: <urlopen error [Errno 101] Network is unreachable>