openstack: use ceph_keyring_permissions by default
openstack: use ceph_keyring_permissions by default

Otherwise this task fails if no permission is set on the item.
Previously the code omited the mode parameter if it was not set, but
this was lost with commit ab370b6ad823e551cfc324fd9c264633a34b72b5.

Signed-off-by: Gaudenz Steinlin <37adcd35bcc1272df00ab3937c03486436bfeb75@cloudscale.ch>

ceph/ceph-ansible,ceph/ceph-ansible