Fix a flake in sshutil.
Fix a flake in sshutil.

We have to wait for the copy to finish before returning, otherwise the readers might go out of scope.
https://travis-ci.org/kubernetes/minikube/builds/132822322

mikegrass/minikube,thatdocslady/minishift,cu12/minikube,dlorenc/minikube,thatdocslady/minishift,warmchang/minikube,LalatenduMohanty/minishift,aaron-prindle/minikube,kubernetes/minikube,kubernetes/minikube,aaron-prindle/minikube,cu12/minikube,kubernetes/minikube,luxas/minikube,aaron-prindle/minikube,Preeticp/minishift,dlorenc/minikube,r2d4/minikube,dlorenc/minikube,dims/minikube,r2d4/minikube,endocode/minikube,dlorenc/minikube,dongsupark/minikube,kubernetes/minikube,dalehamel/minikube,praveenkumar/minishift,skipjaq/minikube,budhrg/minishift,luxas/minikube,kubernetes/minikube,skipjaq/minikube,jimmidyson/minishift,skipjaq/minikube,warmchang/minikube,dongsupark/minikube,endocode/minikube,praveenkumar/minishift,dlorenc/minikube,kubernetes/minikube,dlorenc/minikube,warmchang/minikube,praveenkumar/minishift,minishift/minishift,endocode/minikube,Preeticp/minishift,abbytiz/minikube,minishift/minishift,thatdocslady/minishift,dims/minikube,dalehamel/minikube,budhrg/minishift,kubernetes/minikube,dlorenc/minikube,budhrg/minishift,dongsupark/minikube,warmchang/minikube,jimmidyson/minishift,warmchang/minikube,dlorenc/minikube,cu12/minikube,LalatenduMohanty/minishift,jimmidyson/minishift,mikegrass/minikube,mikegrass/minikube,warmchang/minikube,Preeticp/minishift,LalatenduMohanty/minishift,abbytiz/minikube,dims/minikube,r2d4/minikube,minishift/minishift,warmchang/minikube,dalehamel/minikube,abbytiz/minikube