Update prow to v20190813-2e6c6ee18, and other images as necessary.
Update prow to v20190813-2e6c6ee18, and other images as necessary.

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