rgw/s3: CreateBucket extension for layout type and shard count

rgw/s3: CreateBucket extension for layout type and shard count

extend s3's CreateBucketConfiguration [1] with a custom BucketIndex element
that can override rgw's default bucket index type and shard count

to create an indexless bucket:
  <CreateBucketConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
    <BucketIndex>
      <Type>Indexless</Type>
    </BucketIndex>
  </CreateBucketConfiguration>

to create a normal pre-sharded bucket:
  <CreateBucketConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
    <BucketIndex>
      <Type>Normal</Type>
      <NumShards>1023</NumShards>
    </BucketIndex>
  </CreateBucketConfiguration>

[1] https://docs.aws.amazon.com/AmazonS3/latest/API/API_CreateBucket.html#API_CreateBucket_RequestSyntax

Signed-off-by: Casey Bodley <cbodley@redhat.com>