Adding download_item to file_container_client to download individual …

[maximvelichko]

…file from an artifact

To download a single file from a build's artifact:

        artifact = self.build_client.get_artifact(project_name, build_id, 'artifact_name')

        with open('/tmp/out.zip', 'wb') as f:
            for x in self.file_container_client.download_item(artifact.resource.data.split('/')[1], item_path='artifact_name/path/to/out.zip'):
                f.write(x)