osd: make scrub chunk size tunable

It was hard-coded at 5.  Make it range from 5-15 by default, for now.

We should still keep this smallish since this range is locked for the
duration of the scrub on this chunk.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #350 from ceph/wip-osd-scrub-chunk

Reviewed-by: Samuel Just <sam.just@inktank.com>