rgw: fix bucket placement assignment

When we set bucket.instance meta, we need to set
the correct bucket placement to the bucket (according to
the specific placement rule). However, it might be that
bucket placement was never configured and we just go by
the defaults, using the old legacy pools selection.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: handle ENOENT when listing bucket metadata entries

Just return success (with an empty list)

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #438 from yehudasa/wip-rgw-next

Fix an issue with bucket placements and with listing on new installations.

Reviewed-by: Greg Farnum <greg@inktank.com>