On Ubuntu 16.04, the default networks interface is not always called
eth0, depending on the hardware. The name of the default network
interface on some VM types on AWS is ens3, for example.

Determine the interface name using ip route command instead of
hardcoding to eth0.

Also, /etc/network/interfaces.d/<interface-name>.cfg files no longer
exist in Ubuntu 16.04. Use a script in /etc/network/if-up.d to
adjust the MTU value at startup instead.