do not render systemd.link files
When fixing bug 1579130, we made cloud-init rename devices itself,
rather than relying on the systemd.link files to do that.
That was necessary to
- rename devices in a container
- rename devices on first boot or in any situation when the
link files in the initramfs were stale.
However, cloud-init was still writing .link files like:
/etc/systemd/network/50-cloud-init-ens2.link
That leads to just a confusing situation as cloud-init will trump
any renaming systemd does in all cases.
Also added here is a header into the rendered ENI file:
/etc/network/interfaces.d/50-cloud-init.cfg
that describes how to disable cloud-init networking.