support for self-hosted s3 instances

[felixevers]

Currently its not possible to change the endpoint url for the S3Driver. I would suggest something like this:

    @property
    def s3(self):
        return self.session.resource(
            service_name="s3",
            region_name=self.region,
            endpoint_url=self._endpoint_url,
            use_ssl=self._endpoint_use_ssl,
            verify=self._endpoint_verify, 
        )

[felixevers]

related to #85 but workaround doesnt work for different permission systems applied on s3