mgr/balancer: blame if upmap won't actually work

mgr/balancer: blame if upmap won't actually work

With automatic balancing on, and if mode is set to upmap,
balancer will fail silently if min_compat_client is lower than
luminous.
You can't figure out that unless you take a closer look at the
mgr log, which is super annoying..

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit c7356c66b0b0bb1a8a6197f84b26e6db0d192f60)

Conflicts:
- path: qa/suites/rados/thrash/d-balancer/upmap.yaml
  comment: changeset here -> qa/suites/rados/thrash/d-require-luminous/at-mkfs-balancer-upmap.yaml
- path: src/pybind/mgr/balancer/module.py
  comment: 'set_module_option()' -> 'set_config()' for mgr config-key