Support for Dask Gateway clusters from config

Right now, IIUC, to create a cluster using the button the config takes a python class, args, and kwargs to create the cluster. This isn't flexible enough for dask-gateway, which requires creating an intermediate Gateway object.

Two options

1. Expand the logic of the lab extension's cluster creation to take a snippet of code to run.
2. Update `dask-gateway` to have a "simple" way of creating a cluster that just uses the defaults (cc @jcrist).

https://github.com/dask/dask-gateway/issues/55 is related, but more focused on expanding dask-labextension to take advantage of dask-gateway