openstack: clear the buildpackages directory on stop
openstack: clear the buildpackages directory on stop

When running /etc/init.d/teuthology stop, all OpenStack resources are
destroyed, including the instance hosting the repository where the
buildpackages task artefacts are archived. Remove /tmp/stampsdir so that
everything gets rebuilt.

Signed-off-by: Loic Dachary <b6eae37fc74638590e3f9030895d42446505d41b@dachary.org>

dmick/teuthology,dmick/teuthology,caibo2014/teuthology,SUSE/teuthology,robbat2/teuthology,dreamhost/teuthology,ceph/teuthology,dmick/teuthology,caibo2014/teuthology,ktdreyer/teuthology,SUSE/teuthology,SUSE/teuthology,ceph/teuthology,dreamhost/teuthology,robbat2/teuthology,ktdreyer/teuthology