added mutex lock over teePrefix()
added mutex lock over teePrefix()

warmchang/minikube,kubernetes/minikube,kubernetes/minikube,dalehamel/minikube,kubernetes/minikube,kubernetes/minikube,kubernetes/minikube,kubernetes/minikube,dalehamel/minikube,warmchang/minikube,warmchang/minikube,kubernetes/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,warmchang/minikube,dalehamel/minikube