Get machine info from DNS TXT RR

[ctyler]

It would be useful if the machine list for the infra pane came from a DNS TXT record, so that there's a single point for machine configuration.

For straight pings, listing hostnames would be enough; for ssh checks, we'd need additional info, maybe "displayname=ssh://hostname:port" e.g. archie=ssh://ehl.cdot.systems:2200" where the name displayed on the dashboard would be aarchie and the ssh check would be tested on the specified IP and port.