ospurge crashes sometimes when many incremental backups are deleted

[Simon Reinkemeier]

When a lot of incremental backups exist, ospurge sometimes crashes. This seems to be because the runner tries deleting a backup without first waiting for the deletion of all dependent backups. This causes Openstack to return an error and ospurge aborts the whole process.

I have solved this issue in a personal repository by waiting for backup deletion if another backup for a volume is currently being deleted. See here: https://github.com/simonre/ospurge/blob/master/ospurge/resources/cinder.py

I would be happy to merge the fix into ospurge but I haven't worked with gerrit before and I'm not sure where to go from here.