Configurable base_resource_url and pass to S3 presigned url generation

 - Make vhost or fqdn url generation configurable
 - Pass on vhost/fqdn when generating S3 presigned url for checksums

If base_resource_url is set to the atom, 'host_header' (default) then
the URL will use the Host: header value.

Otherwise, it will use the url set for base_resource_url

Further, pass this value to the S3 presigned URL generation.