openstack: update CentOS image
openstack: update CentOS image

Get the latest CentOS 7 cloud image.

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

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