osd_heartbeat_grace from 20 -> 40 when running valgrind

This will mask occasional 'wrongly marked me down' errors when valgrind
is running (it makes things generally slow).

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #37 from ceph/wip-heartbeat

osd_heartbeat_grace from 20 -> 40 when running valgrind