tests: set pool size to 1 in ceph-override.json

tests: set pool size to 1 in ceph-override.json

setting this setting to 1 makes the CI covering the related code in the
playbook without breaking the upgrade scenarios.

Those scenarios were broken because there is a check `TASK [waiting for
clean pgs...]` in rolling_update.yml, since the pool size for
`cephfs_metadata` and `cephfs_data` are updated to `2` in
`ceph-override.json` and there is not enough osd to honor this size,
some PGs are degraded and make the mentioned check failing.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 3ac6619fb9aa0ea29041ce122b4dfac9a51fc235)