Code review comment for ~mpontillo/maas:beaconing-service

I tried to make this branch as small as possible so that it can be reviewed and landed; I've also developed a command to test beaconing, but that is not part of this proposal. That branch can be found here:

https://code.launchpad.net/~mpontillo/maas/+git/maas/+ref/send-beacons-command

For now, MAAS listens for beacons on any interface that is configured for network monitoring (that is, passive ARP observation). This approach may change in the future.

MAAS does not take any action on the received beacons except to (1) reply to any solicitation beacons with advertisement beacons and (2) keep state about beacons received within +/- two minutes of the current system time. This is enough to allow testing, sets the stage for MAAS to make some assumptions about fabric connectivity. But those assumptions are not yet acted upon in this branch.