platforms: remove all nodes from a platform

[oliver-sanders]

rose host-select allows us to remove all hosts from a group which can be a good way of draining a platform from a simple config file rather than having to do something more substantial.

We should support this in Cylc somehow, say by allowing platforms with no hosts or via a separate configuration.

See #3887 (comment)

[dpmatthews]

See also #2698.
It might be good to hold tasks until the platform becomes available rather than fail/retry? (but there would need to be notification)