f28 container build broken by tripleo-repos InvalidArguments('Branches only suppported with centos7')

2019-04-02 14:30:38.120281 | primary | + REPO=tripleo-ci-testing
2019-04-02 14:30:38.120347 | primary | + tripleo-repos -b stein --mirror https://mirrors.fedoraproject.org --rdo-mirror https://trunk.rdoproject.org tripleo-ci-testing
2019-04-02 14:30:38.261160 | primary | WARNING: --centos-mirror was deprecated in favour of --mirror
2019-04-02 14:30:38.261645 | primary | Traceback (most recent call last):
2019-04-02 14:30:38.261701 | primary | File "/home/zuul/venv/bin/tripleo-repos", line 10, in <module>
2019-04-02 14:30:38.261721 | primary | sys.exit(main())
2019-04-02 14:30:38.261787 | primary | File "/home/zuul/venv/lib/python3.6/site-packages/tripleo_repos/main.py", line 344, in main
2019-04-02 14:30:38.261814 | primary | base_path = _get_base_path(args)
2019-04-02 14:30:38.261867 | primary | File "/home/zuul/venv/lib/python3.6/site-packages/tripleo_repos/main.py", line 232, in _get_base_path
2019-04-02 14:30:38.261922 | primary | raise InvalidArguments('Branches only suppported with centos7')
2019-04-02 14:30:38.261970 | primary | tripleo_repos.main.InvalidArguments: Branches only suppported with centos7

http://logs.rdoproject.org/openstack-periodic/git.openstack.org/openstack-infra/tripleo-ci/master/periodic-tripleo-fedora-28-stein-containers-build-push/5676c10/job-output.txt.gz

related review:
https://github.com/openstack-infra/tripleo-ci/commit/8a60923e841c080ba03f86eb8b80179b788c7b6e