rgw/multisite: in a multisite env with bucket sync policies configured,

rgw/multisite: in a multisite env with bucket sync policies configured,
we may end up orphaning objects on remote zones when a delete bucket
is issued on metadata master. to avoid this, list the buckets on remote
zones and delete bucket only when empty. if a zone is unreachable we
drop that zone and continue with bucket deletion. such zones might have
orphaned objects that will have to be cleaned up using radosgw-admin tool

Signed-off-by: Shilpa Jagannath <smanjara@redhat.com>