closes #137

Turned out that having a spawner as a plugin to a spawner was a little tricky as there's all sorts of setup Jupyterhub does.

We could also use the existing SSH spawner plugin by subclassing and overriding the choose_remote_host method and bolting on some bits. This plugin is a little over the top for us as it does file transfers etc.

Simple solution, runs the command through an SSH connection, similar to how cylc play does it.

Note: JupyterHub spawners run as the user so this spawner, the SSH connection and the UIS is spawns are all in userland even if the hub is not.

Here's the configuration to use it:

c.JupyterHub.spawner_class = 'cylc.uiserver.distributed_spawner.DistributedSpawner'
c.DistributedSpawner.hosts = ['localhost']  

Unfortunately the UIS doesn't like being spawned onto other hosts, I think we need to patch the handlers with the location of the Hub. I've tried patching the MainHandler but that leaves me with the following error (Hub stdout):

tornado.application WARNING  oauth state '<key>' != None

So ATM this can't be used to spawn UIS on other hosts which makes it a bit of a chocolate teapot.

Been looking at the JupyterHub single-user thinggy trying to figure out how to fix it. They configure their handlers quite differently (redirect rather than render) and seem to provide hub-host and hub-prefix CLI opts?

https://github.com/jupyterhub/jupyterhub/blob/3fec19d191119e4203c243f44c43e74c8fe62f04/jupyterhub/singleuser/mixins.py#L141-L155

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