KVM guests do not get their Juju interfaces file if interface names are incorrect

Juju writes out `/etc/network/interfaces-juju` with the required network configuration, as part of user-data, then tries to use this file to configure networking. If successful, `interfaces-juju` replaces `interfaces`.

This is failing on xenial, at least, because it assumes to use eth0..ethN as interface names, but these interfaces don't exist.

The result is that the default cloud-init interfaces.d config file is left in use (DHCP on ens3, in the xenial case).

The KVM template is also assuming/trying to use eth names as interface guest dev values, but they are not appearing in the guest.

Bug 1646063 will probably need to be fixed before this can be properly resolved for multi-homed machines.