2 minor changes to fix Ubuntu 14.04 compatibility

1) Newer versions of Ubuntu no longer support 'service networking stop/start',
instead preferring 'ifdown/up -a'. This command also works on older versions
of Ubuntu and on Debian, so changed exec reference to use it*.

*Initially I went with a long case statement (as you can't have selectors within
selectors) to only change to this method on Ubuntu 14.04 and above. If you
would prefer this to always using the new method on Debian/Ubuntu, I can
reinstate it.

2) Notifying when the interface definition *fragment* changes causes the
notify to occur before the actual full file has been written, meaning that
interfaces get bounced to pick up new settings before those new settings can
be obtained, and are not bounced after said settings have been written. Moved
the notify from the interface concat::fragment to the main concat file.
Notification now correctly occurs after file has been written on Debian/Ubuntu
systems.
If /sbin/ifdown -a && /sbin/ifup -a work well it's ok to keep them also for the older versions