Right now, preload isn't working because it thinks that the
kubernetes/dashboard images don't exist. This is because they now have a
docker.io prefix introduced by 4c54e786

Since `docker images` won't return image names with that prefix, we want
to strip the images of `docker.io` before checking if they exists in the
docker daemon.

This also fixes the TestPause/serial/SecondStartNoReconfiguration
failing integration test.