backup: retry a lot because it fails often
tox -e backup
tests/run-tests.sh tox -e py3 -- --log-cli-level=INFO -k test_libvirt_backup_create_pet_upload tests/enough/common/test_libvirt.py
In the past weeks it almost always failed twice for a given glance upload. And it always succeeded within the day: it is worth insisting.
Fixes: #338 (closed)