Better non-Amazon S3 support (including Ceph)

- Hostname was not overwritten in validation.
- Support older S3 path-style access //$HOSTNAME/$BUCKET/
- Make it possible to entirely disable S3 if your provider does not
  support it.

Confirmed working with Ceph RADOSGW v0.77-1raring.

Signed-off-by: Robin H. Johnson <rjohnson@sitka.bclibraries.ca>