Create link between WrapSpawner and its child spawner traitlets

[cmd-ntrf]

This PR addresses issues #24, #25 and #26.

It uses traitlets.directional_link[1] to make any change done on WrapSpawner's traitlets also done on the child spawner's traitlets for the traitlets that both classes share.

#25 - current_port is not an attribute of WrapSpawner, but I will also submit a patch for BatchSpawner that remove that traitlet in favor of only using port.

[1] https://traitlets.readthedocs.io/en/stable/utils.html#traitlets.directional_link

[mbmilligan]

@cmd-ntrf or @rkdarst - since we don't have automated testing for Wrapspawner, what I would like to see is confirmation that this works with: a) both Jupyterhub 0.9 and 1.0, and b) with a couple of different spawners, not just Batchspawner.

Basically, I am nervous about the blanket forwarding of like-named traits - that's something I had specifically avoided doing previously. But if this works for those basic cases, I'm happy to accept it and we'll fix any breakage that comes along later.

[rkdarst]

• Combined with Singleuserapp and 1.0 batchspawner#143, and jhub==0.9.0, this works
• combined with batchspawner=master and jhub==0.9.0, does not work. But that combination doesn't work without this PR either, so...

I don't easily have other spawners to test. Does anyone else?

[mbmilligan]

Felix said he would try to test it with Dockerspawner.

[cmd-ntrf]

After testing, JupyterHub 0.9.6 and 1.0 in combination with the proposed PR work for dockerspawner.SystemUserSpawner and jupyterhub.spawner.LocalProcessSpawner.

This is my Terraform setup for test with JupyterHub 1.0 if someone would like to double check:
https://gist.github.com/cmd-ntrf/95f9843a8dad1684b31a2c16e50dbef5

[mbmilligan]

Excellent, thanks Felix! Merging now.