RFC: have a flag to advertise service groups on local network using multicast DNS or similar

[bixu]

(This might be more of an RFD (Request For Discussion). Full disclosure, I'm not sure if this is a good idea, but I keep thinking about the sharp edges gossip ring setup has for new users.)

What if we allowed Supervisors to find each other using MDNS (this would certainly be a non-default option as MDNS can be quite noisy). The thoughts I've had so far go something like:

hab sup start <origin>/<app> --group <group> --mdns

As the first Supervisor starts, it uses it's namespace to construct and advertise an MDNS record for itself:

0-<app>-<origin>-<group>.local

Later Supervisors 'guess' the MDNS name the first Supervisor and try to use that name as their gossip --peer:

hab sup start <origin>/<app> --group <group> --mdns
hab sup start <origin>/<app> --group <group> --peer 0-<app>-<origin>-<group>.local
1-<app>-<origin>-<group>.local

[rsertelon]

This is similar as what is written in #849

[elliott-davis]

Closing as a duplicate of #849