rgw: implicit rgw_bucket placement, manifest uses rgw_raw_obj

rgw: implicit rgw_bucket placement, manifest uses rgw_raw_obj

Two main changes here:
1. Newly created rgw_bucket does not have a predetermined placement pools
assigned to it. The placement_id param in the objects themselves points
at where the data is located. This affects object's tail location, head
is located where the bucket instance's placement rule points at.
2. Modify object manifest to use rgw_raw_obj instead of rgw_obj.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>