Comment 1 for bug 1225922

Currently networking coming up happens independent of cloud-init. We could potentially make cloud-init local block networking from coming up, but I don't think thats something I'd want to take on for cloud-init 0.7.3 / Ubuntu 13.10.

I'm not really sure how much a hack this is, if at all.
The solution I'd propose would be:
 * use 'ifquery' to get the list of networking interfaces in the original
   from cloudinit import util
   (out, _err) = util.subp(['ifquery', '--list', '--allow', 'auto'])
   orig_list = out.splitlines()

 * use ifquery to get the list of 'auto' interfaces in your new list:
   (out, _err) = util.subp(['ifquery', '--interfaces=/tmp/my.tmp.interfaces',
                            --list', '--allow', 'auto'])
   new_list = out.splitlines()

 * take down interfaces in orig_list and new_list
   for iface in [i for i in orig_list if i in new_list]:
      util.subp(['ifdown', iface]

   # have to ignore errors above in case the interface wasn't up.

 * write/update /etc/network/interfaces file

 * bring all interfaces up:
   util.subp(['ifup', '-a'])

I opened bug 1226067 (http://pad.lv/1226067) to address an issue where
ifquery is busted in saucy, but that will get fixed.