mgr/balancer: restrict automatic balancing to specific weekdays

mgr/balancer: restrict automatic balancing to specific weekdays

We have implemented similar semantics for the scrub behaviour.
Let's try to keep pace with that.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit 0412a51ac6dfecc1a72b654196ecb1a599dcc94d)

Conflicts:
- path: src/pybind/mgr/balancer/module.py
  comment: 'get_module_option()' -> 'get_config()' for mgr config-key
    'begin_weekday' and 'begin_weekday' should be int, but
    luminous/mimic config-key always return string.