tests: Make test_delete_container_versions less flakey
tests: Make test_delete_container_versions less flakey

Hammering that test in a tight loop, I'd often see failures due to
ordering issues. Make the delete single-threaded to avoid that.

Change-Id: Iff45be32a7c3f258214cce78001fd33ad0a39b8c

openstack/python-swiftclient,sohonetlabs/python-swiftclient,sohonetlabs/python-swiftclient,openstack/python-swiftclient