Can't use non aws s3 providers that doesn't support access to bucket by subdomain.

[danielannan]

When base_url is set to an address using a host, the aws sdk will try to access the bucket by bucket.host.com, some non-aws s3 providers doesn't support this. Access by host.com/bucket is supported, but it's currently not possible to force this behavior in the plugin. I added a pull request which adds a setting that uses the s3 client's use_path_style_endpoint option to force using host.com/bucket in these cases.
#483