* If a network tail file is found, append to the generated config.
* This functionality allows users to create a tail file @
/etc/systemd/network/template/dosync-${interface}.network.tail to
add additional options to the network config.
* Example use case: Adding "Tunnel=name-of-tunnel" to the eth0
interface file to signal systemd-networkd to bring up a tunnel using
the eth0 interface.