workunit/cephtool: Use '! cmd' when expecting failure

Signed-off-by: Dan Mick <dan.mick@inktank.com>
ceph: reject negative weights at ceph osd <n> reweight

Check the integer (fixed-point) value to avoid any worries
about floating-point rounding.  Add tests for reweight < 0.

Fixes: #3872
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage.weil@inktank.com>