Add collection of /etc/network/interfaces.d for Ubuntu

Since 14.04 Trusty, ubuntu uses config scripts in
/etc/network/interfaces.d. Collect those.
Closes: #264

Signed-off-by: Louis Bouchard <louis.bouchard@canonical.com>
Merge pull request #265 from karibou/fix_interfaces.d

Add collection of /etc/network/interfaces.d for Ubuntu