mgr/cephadm: autotune osd memory

mgr/cephadm: autotune osd memory

- set osd_memory_target_autotune=true to enable
- tuning is periodic (check every 10m by default)
- tuned values are reflected by osd_memory_target config options scoped
  to the host
- only make a change if it appears that we will affect at least 1 of the
  relevant OSDs
- attempt to clean out conflicting options.  (This is imperfect, since any
  manner of weirdly-scoped config options could be responsible; we only
  attempt to clean out one scoped directly to the osd name.)

Signed-off-by: Sage Weil <sage@newdream.net>