Implementing switch group optimization

_Idea for reducing traffic on network bottlenecks like ethernet switches (, VPNs, routers, firewalls):_

Add a user-configurable string called `peer group`, and make planner favor peer nodes in the same group. The optimization might be significant if there are many peers and large switches.

For routers this could be autodetected (by traceroute,) but for switches a manually configured group name seems to the only way.

The group name could even be hierarchical, domain name style (e.g. `room-b.office-west.building1`) to reduce congestion on backbone switches by further minimizing hops when possible.