Add nat monitor

First version of nat monitor. I've tested and it and it seems to work well. I'm mostly interested in design considerations, because it feels a bit clunky to me.
That's not entirely accurate. This script does not route any traffic, it just monitors the routes. If this script goes down then the routing will still continue just as before. However, no changes to the routing will be made by this machine.

The last problem is easy to solve thanks to the log message "nat-monitor/eu-west-1c: all routes checked successfully." which gets printed every five seconds. We'll create an 'inactivity alarm' in logentries and have it email us when the log message is missing for either availabiliy zone.