Comment 7 for bug 1270881

Works like a charm. ^^

$ cat /etc/network/interfaces
source /etc/network/interfaces.d/*

$ cat /etc/network/interfaces.d/loopback.conf
auto lo
iface lo inet loopback

$ cat /etc/network/interfaces.d/primary.conf
auto eth0
iface eth0 inet dhcp
iface eth0 inet6 auto

$ dpkg -l | grep augeas
ii augeas-lenses 1.1.0-0ubuntu4~ppa1 all Set of lenses needed by libaugeas0 to parse config files
ii augeas-tools 1.1.0-0ubuntu4~ppa1 amd64 Augeas command line tools
ii libaugeas0 1.1.0-0ubuntu4~ppa1 amd64 Augeas configuration editing library and API

$ augtool ls /files/etc/network
interfaces.d/ = (none)
interfaces/ = (none)

$ augtool print /files/etc/network/interfaces
/files/etc/network/interfaces
/files/etc/network/interfaces/source = "/etc/network/interfaces.d/*"

$ augtool ls /files/etc/network/interfaces.d
loopback.conf/ = (none)
primary.conf/ = (none)

$ augtool print /files/etc/network/interfaces.d/loopback.conf
/files/etc/network/interfaces.d/loopback.conf
/files/etc/network/interfaces.d/loopback.conf/auto
/files/etc/network/interfaces.d/loopback.conf/auto/1 = "lo"
/files/etc/network/interfaces.d/loopback.conf/iface = "lo"
/files/etc/network/interfaces.d/loopback.conf/iface/family = "inet"
/files/etc/network/interfaces.d/loopback.conf/iface/method = "loopback"

$ augtool print /files/etc/network/interfaces.d/primary.conf
/files/etc/network/interfaces.d/primary.conf
/files/etc/network/interfaces.d/primary.conf/auto
/files/etc/network/interfaces.d/primary.conf/auto/1 = "eth0"
/files/etc/network/interfaces.d/primary.conf/iface[1] = "eth0"
/files/etc/network/interfaces.d/primary.conf/iface[1]/family = "inet"
/files/etc/network/interfaces.d/primary.conf/iface[1]/method = "dhcp"
/files/etc/network/interfaces.d/primary.conf/iface[2] = "eth0"
/files/etc/network/interfaces.d/primary.conf/iface[2]/family = "inet6"
/files/etc/network/interfaces.d/primary.conf/iface[2]/method = "auto"