add dynamic support for the container constraints for the SwarmSpawner

[drzhouq]

Proposed change

One can specify "static" constraint for the container (for example, c.SwarmSpawner.extra_placement_spec = { 'constraints' : ['node.role==worker'] }) to be spawn by the jupyterhub server. Is it possible to dynamic change the constraint based on image chosen by the user or even the user name?

Alternative options

Who would use this feature?

Some images need to use GPU, and not every computer in a swarm has GPU. Enable dynamic constraints for the container will help to ensure the container spawn can have access to the GPU.

(Optional): Suggest a solution

It seems that c.Spawner.pre_spawn_hook is currently too late to apply the constraint to container spawn, but it should be the logic place to implement this "dynamic" container constraint.

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[minrk]

Indeed, most of these extra_ fields should be allowed to be callables. Starting on this in #500, swarm should come next