Disable predictable interfaces naming in stretch

In Stretch release predictable interfaces are enabled by default:

Since these images will be used for virtual machines and will be
provisioned with snf-image which configures /etc/network/interfaces
we disable predictable interfaces.

Disabling can be implemented in one of two ways:

• Put "net.ifnames=0" into the kernel command line (e. g. in /etc/default/grub's GRUB_CMDLINE_LINUX_DEFAULT, then run "update-grub").

• Disable the default *.link rules with "ln -s /dev/null /etc/systemd/network/99-default.link" and rebuild the initrd with "update-initramfs -u".

This commit implements the second.

[1] https://www.debian.org/releases/stretch/amd64/release-notes/ch-whats-new.en.html#new-interface-names
[2] https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/