osd: add mon_reweight_max_osds to limit reweight-by-* commands

osd: add mon_reweight_max_osds to limit reweight-by-* commands

Add configurable mon_reweight_max_osds which limits the number
of OSDs modified each time reweight-by-* is called (by default
to 4 OSDs).

Also change the order in which we look at OSDs to go from most
to least utilized.

Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>
(cherry picked from commit 3aa0ccefddc1790df5b808bfccd5da4857951220)