Disable dns support for buckets when "host_bucket" hostname doesn't have a %(bucket)s item in it.

Here is my proposal to support S3 servers that don't support usage of dns hostname for buckets like Ceph:
In the config, currently there is the host_bucket entry that give the bucket "hostname" pattern.
Ex.: host_bucket = "%(bucket)s.s3.amazonaws.com"
With my patch, if you remove the bucket part and only use the base url like that:
Ex.: host_bucket = "s3.amazonaws.com"
Then, dns resolution will be disabled and "path-style" bucket access be used.