Support Umbrella forwarding

[anarkiwi]

@mbruyere

We need to support Umbrella (http://www.touix.net/en/content/tousix-project-technical-overview).

Umbrella essentially hard codes broadcast to unicast (where unicast destinations are all known in advance, having been provisioned). This can be accomplished with ACLs.

However we also need the failover functionality of groups (we will support a port failover option using groups).

[anarkiwi]

This is deployed and working, excepting failover groups which is being tracked in a separate issue, #801