HA router setup

[samcday]

Related to #645 + #646

#646 means I'll soon have an L2 wifi backhaul to span the AZs. Other than the "portal peer" (the one with the WAN upstrem) being slightly special via the mesh11sd machinery, these are just a bunch of equal peers hanging out.

As a first pass for that work, I would just manually designate one of the routers the "primary", and it's the one that runs dhcpd+dnsmasq+FRR.

But, I could opt for a HA setup instead.

So per that guide, use keepalived+VRRP to elect one of the online routers as the primary, which then receives the dhcp+dns+BGP traffic.

I also wonder if, since I already need FRR in the stack for the BGP announcements coming from the cluster, this might be useful here as well. Not sure exactly how just yet, but I guess it would be something involving all 3 nodes running FRR, accepting advertisements and distributing them amongst each other.