rgw: fix set_buckets_enabled(), set_bucket_owner()

Fixes: 5808

We cannot call get_bucket_instance_info() at that point,
as the bucket structure wasn't initialized, so we don't
have the bucket instance location information. Just calling
get_bucket_info().

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>