Allow config to specify the bucket exists

In the 99% case the bucket is just always there. And if it is not the
read/write will fail hard anyways. Esp on big instances the Objectstore
is not always fast and this can save a few hundered ms of each request
that acess the objectstore.

In short it is adding

'verify_bucket_exists' => false

To the S3 config part

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>