Remove show-libmachine-logs flag

[r2d4]

I'm proposing that we remove the --show-libmachine-logs flag and set that behavior by a certain verbosity level --v=6. I don't think there are many situations where we want debug level minikube logs but not libmachine logs. I'm arbitrarily suggesting glog.V(6) as the cutoff for libmachine logs.

If you guys think this is a good idea, I'll go ahead and make this change eventually or someone can pick it up