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.
A shared PID namespace were enabled by default in the 1.7 when running
with a supported Docker runtime, but a Docker version that supports
a shared namespace was not qualified for use.
Release 1.8 will qualify a docker version supporting shared PID, but we
don't want to cause disruption for container images which expect always
to have PID 1.
