DHCP detection when network config is split across multiple files

As a result of [Ubuntu bug #1262951](https://bugs.launchpad.net/ubuntu/+bug/1262951), /etc/network/interfaces may not contain the complete network configuration. External config files can now be sourced from that file.

That was the setup on some of our 12.04 servers, so the `dhcp_enabled` fact was incorrectly reporting `false`.

This change updates the custom `dhcp_enabled` fact to check /etc/network/interfaces *and* any external files sourced from that file.

Here's the configuration from one of our servers which was failing before (`dhcp_enabled` was reporting `false`):

````
ubuntu@server:~$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 12.04.4 LTS
Release:	12.04
Codename:	precise
````
````
ubuntu@server:~$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# Source interfaces
# Please check /etc/network/interfaces.d before changing this file
# as interfaces may have been defined in /etc/network/interfaces.d
# NOTE: the primary ethernet device is defined in
# /etc/network/interfaces.d/eth0
# See LP: #1262951
source /etc/network/interfaces.d/*.cfg
````
````
ubuntu@server:~$ cat /etc/network/interfaces.d/eth0.cfg 
# The primary network interface
auto eth0
iface eth0 inet dhcp
````
Oh, huh. You're right. At one point I was ensuring only the top-level `interfaces` file would be checked for included files, but it looks like I dropped that accidentally. I'll put that back.