radosgw-admin: accept negative values for quota params

and document that in the usage output.

Fixes: #6621

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #778 from ceph/wip-6621

radosgw-admin: accept negative values for quota params

Reviewed-by: Sage Weil <sage@inktank.com>