Stop supporting the "nsenter" exec handler. Only the Docker native exec
handler is supported.
The flag was deprecated in Kubernetes 1.6 and is safe to remove
in Kubernetes 1.9 according to the deprecation policy.
Module remotecommand originally part of kubernetes/pkg/client/unversioned was moved
to client-go/tools, and will be used as authoritative in kubectl, e2e and other places.
Module remotecommand relies on util/exec module which will be copied to client-go/pkg/util