debian: properly configure interfaces

interfaces are not configured at all because specific interface files
are created but not sourced. This will result in the VMs running with
the loopback interface only

This is a regression, the problem was already fixed in the past, see commit
b822581d88

Change-Id: I846642772ab582d7567e5182c860cfa0fe882a15

elements/debootstrap/install.d/10-debian-networking

@@ -26,6 +26,7 @@ echo $DISTRO_NAME > /etc/hostname
 
 # cloud images expect eth0 and eth1 to use dhcp.
 mkdir -p /etc/network/interfaces.d
+echo "source /etc/network/interfaces.d/*" >> /etc/network/interfaces
 for interface in eth0 eth1; do
     cat << EOF | tee /etc/network/interfaces.d/$interface
 auto $interface