idea: use host constraint for queues instead of properties

[grondo]

An annoying property (no pun intended) of our current queues config is that the associated resources in a queue are based on a property constraint. This requires properties be assigned in the resource config section, and also means that updating a queue requires moving properties around which currently requires a system restart.

Based on some offline discussion with @garlick, now that Fluxion supports hostlist constraints, we could instead specify a set of hosts explicitly in the queue config. This would decouple queue and resource configs, and would allow queue updates without a restart, or even a flux queue update command to remove/add hosts to queues dynamically for the creation of temporary queues.

If the hostlist constraint was a required attribute of the queue config, then flux queue list could easily display hosts in the queue which would be nice.