Packages may auto-create directories in /etc/kubernetes, and users also
need files such as cloud-config.json to be present and preserved at
their default locations in /etc/kubernetes. As such this modifies
pre-flight checks to only require the absence of the files and
directories we explicitly create in kubeadm.

Reset is similarly modified to not wipe out /etc/kubernetes entirely.
When resetting directories we also now preserve the directory itself,
but delete it's contents.

Also adds tests for reset command logic specifically for /etc/kubernetes
cleanup, to ensure user files are not inadvertently wiped out.