rados: add set-alloc-hint command

Set allocation hint for an object.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
qa: add script for testing rados allocation hint

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1400 from ceph/wip-hint-tests

wip-hint-tests

Reviewed-by: Sage Weil <sage@inktank.com>