openstack: centos-7 image must be recent

openstack: centos-7 image must be recent

Using the CentOS 7 image available at

  http://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud.qcow2

ansible will think it runs

  RedHat 7.1.1503 (Core)

and with a more recent image

  http://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud-1503.qcow2

it will think that it runs

  CentOS Linux release 7.1.1503 (Core)

http://tracker.ceph.com/issues/12725 Fixes: #12725

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