osd: add 'osd heartbeat min healthy ratio' tunable

This was hard-coded to 1/3; make it tunable.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #533 from ceph/wip-osd-healthy-tuanble

osd: add 'osd heartbeat min healthy ratio' tunable

Reviewed-by: Samuel Just <sam.just@inktank.com>