Use fixtures.TempDir directly.

Change-Id: I02e20156af20cfc019262f39e343e3ec1ce06a85
Reviewed-on: https://review.openstack.org/35541
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins