resource: add option to allow R to be updated

[garlick]

Problem: when brokers are dynamically added to an instance as proposed in #5184, there is no clean way to combine the resources provided by the enclosing instance with dynamically discovered ones. Dummy resources have to be provided in advance and the resource module must be loaded with the noverify option.

Two ideas are:

1. add an option so that if the instance size exceeds the number of nodes in R, a heuristic is used to automatically add dummy resources for the missing nodes. For example, replicate one of the nodes provided by the enclosing instance.
2. add an option to update R based on hwloc when resource verification fails on a node (as an alternative to noverify)

Further work would be needed on RFC 28 to add some way to notify the scheduler of resource changes. Meanwhile, as suggested by @grondo, the scheduler could simply be reloaded after all the resources are online for some use cases.