Network setup on Ubuntu Bionic

[s-fu]

Ubuntu 18.04 uses netplan instead of ifupdown to manage the network. snf-image fails to pass correct configuration to the VM, because:

1. /etc/debian_version is checked and if it exists, snf-image will assume the VM uses ifupdown. ifupdown.sh.in will be called.
2. if cloud-init is enabled and /etc/debian_version does not exist, cloud-init.sh.in will be called. However, it only includes version 1.

The workaround is to install ifupdown on the VM before creating the template.