swap non-periodic kubeadm job for periodic in release-1.7-blocking
swap non-periodic kubeadm job for periodic in release-1.7-blocking

the periodic job provides high frequency results, and exercises the
same code

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