This change adds a new --rootfs=path option to kubeadm, and (if
provided) chroot()s to this path before performing file operations.

This makes it possible to run the kubeadm binary from a container, but
perform remaining file operations against the host filesystem using
something like:

    docker run -v /:/rootfs --net=host --uts=host --pid=host \
       kubeadm:latest init --rootfs /rootfs...

Fixes kubernetes/kubeadm#503