Update prow to v20190815-f8ab81959, and other images as necessary.
Update prow to v20190815-f8ab81959, and other images as necessary.

GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid