osd: add sanity check/warning on a few key configs

Warn when certain config values are set to bad values.

Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1964 from ceph/wip-osd-configs

osd: add sanity check/warning on a few key configs

Reviewed-by: Loic Dachary <loic@dachary.org>