Tear down fix merged from master

As we're still working with the grizzly release, the bug fix from yfried-redhat makes sense for our tests:
https://github.com/openstack/tempest/commit/ca1d9a9523b9d6615a8db9f785144e22adb8c81b#diff-7e531e6377f9e9e1d3862fe5a5f2229d