Can't set environment variables for dask-gateway

[scottyhq]

We can do this on Pangeo Hubs, but not Binder currently.

Seems like it's just a straightforward copy of the following
https://github.com/pangeo-data/pangeo-cloud-federation/blob/a2d6c9a31a7a08c27275c4df6c779aba7c2c4c65/pangeo-deploy/values.yaml#L124-L131

to values.yaml in this repo

pangeo-binder/pangeo-binder/values.yaml

Line 240 in 6b4a752

optionHandler: |

@TomAugspurger - am I missing anything? I think some Pangeo Gallery examples will likely depend on this

[TomAugspurger]

Yep I think that's all there is to it.

Just need to provide the Mapping option and then make sure that it's in the dict returned https://github.com/pangeo-data/pangeo-cloud-federation/pull/762/files.