mon [stretch-mode]: Allow a max bucket weight diff threshold

mon [stretch-mode]: Allow a max bucket weight diff threshold

Problem:
Users ran into a problem where the crush bucket
weight different check in stretch mode is too strict, e.g.,
one of the disk that is added to one of the node had slight variation
in the capacity and this caused ceph to fail from enabling the stretch
cluster because crush weight is not balanced. The difference was very small.

Solution:
- Introducing: mon_stretch_max_bucket_weight_delta in mon.yaml.in
  this config var is default to 0.1 and is used as a threshold
  to allow the difference between the two crush buckets in stretch mode
  to be no greater than 10%.
- Introducing: STRETCH_MODE_BUCKET_WEIGHT_IMBALANCE as health warnings
  when the weight delta between the two sites exceeds 10%
- Modified documentations
- Modified tests that exercises this code path

Fixes: https://tracker.ceph.com/issues/72994
Signed-off-by: Kamoltat Sirivadhna <ksirivad@redhat.com>