Instead of hardcoding `luminous`, use the `ceph_stable_release` variable
to point to the correct repository.

This is now uncommented in roles/ceph-defaults/defaults/main.yml to be
available, as it is only used if ceph_repository is set to 'obs'.

group_vars/*.sample files have been regenerated using the
./generate_group_vars_sample.sh script.

Signed-off-by: Johannes Kastl <kastl@b1-systems.de>
(cherry picked from commit 0cedc4d3)