mon: make reweight-by-* sanity limits configurable

Also drop the somewhat redundant osd_sum.kb check; the main thing we care
about here is

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2287 from ceph/wip-reweight-tunables

mon: make reweight-by-* sanity limits configurable

Reviewed-by: Greg Farnum <greg@inktank.com>