Make KeyValueStore support set_alloc_hint op

Add a new config let KeyValueStore support configurable strip size.
set_alloc_hint op can affect the strip size of the specified object
and the expect write size will become the strip size of the object.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Add set_alloc_hint test to ceph_test_objectstore

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>